diff --git a/lib/CamlPaml/CamlPaml.mlpack b/lib/CamlPaml/CamlPaml.mlpack
index 5900b58..5ff9a0a 100644
--- a/lib/CamlPaml/CamlPaml.mlpack
+++ b/lib/CamlPaml/CamlPaml.mlpack
@@ -4,10 +4,10 @@ NewickLexer
 T
 P
 Q
-Infer
 Expr
 Fit
 PhyloModel
+PhyloLik
 PhyloEM
 Code
 Parsimony
diff --git a/lib/CamlPaml/CamlPaml.odocl b/lib/CamlPaml/CamlPaml.odocl
index 05b1248..d5fbe19 100644
--- a/lib/CamlPaml/CamlPaml.odocl
+++ b/lib/CamlPaml/CamlPaml.odocl
@@ -2,10 +2,10 @@ Code
 Newick
 T
 Parsimony
-Expr
+PhyloModel
 P
 Q
-PhyloModel
+Expr
+PhyloLik
 PhyloEM
-Infer
 Fit
diff --git a/lib/CamlPaml/PhyloEM.ml b/lib/CamlPaml/PhyloEM.ml
index 12abdba..2bdfeb0 100644
--- a/lib/CamlPaml/PhyloEM.ml
+++ b/lib/CamlPaml/PhyloEM.ml
@@ -11,12 +11,12 @@ let new_sufficient_statistics m =
 	let t0 = PM.tree m
 	Array.init (T.size t0 - 1) (fun _ -> Array.make_matrix n n 0.)
 
-let collect_sufficient_statistics m leaves ss =
-	let inter = PM.infer m leaves
-	let lik = Infer.likelihood inter
+let collect_sufficient_statistics ?workspace m leaves ss =
+	let inter = PM.prepare_lik ?workspace m leaves
+	let lik = PhyloLik.likelihood inter
 	let t = PM.tree m
 	for i = 0 to T.root t - 1 do
-		Infer.add_branch_posteriors inter i ss.(i)
+		PhyloLik.add_branch_posteriors inter i ss.(i)
 	lik
 	
 let clean_sufficient_statistics ?(tol=1e-6) ss =
diff --git a/lib/CamlPaml/PhyloEM.mli b/lib/CamlPaml/PhyloEM.mli
index adc3927..29280a1 100644
--- a/lib/CamlPaml/PhyloEM.mli
+++ b/lib/CamlPaml/PhyloEM.mli
@@ -8,8 +8,8 @@ type sufficient_statistics = float array array array
 
 val new_sufficient_statistics : PhyloModel.t -> sufficient_statistics
 
-(** compute the E-step statistics for one site, given the leaves (as would be given to {{:PhyloModel}PhyloModel.infer}). Update the given [sufficient_statistics] and return the probability of the leaves under the model. This should be called for each site to collect the statistics for the whole alignment. *)
-val collect_sufficient_statistics : PhyloModel.t -> Infer.leaf array -> sufficient_statistics -> float
+(** compute the E-step statistics for one site, given the leaves (as would be given to {{:PhyloModel}PhyloModel.prepare_lik}). Update the given [sufficient_statistics] and return the probability of the leaves under the model. This should be called for each site to collect the statistics for the whole alignment. *)
+val collect_sufficient_statistics : ?workspace:PhyloLik.workspace -> PhyloModel.t -> PhyloLik.leaf array -> sufficient_statistics -> float
 
 (** any entry in the sufficient statistics less than [tol] is set to zero. useful if e.g. planning to compress the sufficient statistics for transport between compute nodes *)
 val clean_sufficient_statistics : ?tol:float -> sufficient_statistics -> unit
diff --git a/lib/CamlPaml/Infer.ml b/lib/CamlPaml/PhyloLik.ml
similarity index 79%
rename from lib/CamlPaml/Infer.ml
rename to lib/CamlPaml/PhyloLik.ml
index 40f9053..76f6d15 100644
--- a/lib/CamlPaml/Infer.ml
+++ b/lib/CamlPaml/PhyloLik.ml
@@ -18,21 +18,29 @@ let get_leaf k leaf = match leaf with
 	| `Distribution ar -> Gsl_vector.of_array ar
 	| `Marginalize -> hashcons_raw_marg k
 
+type workspace = {
+	mutable generation : int;
+	data : Gsl_matrix.matrix
+}
+
 type intermediate = {
 	tree : T.t;
 	pms : Gsl_matrix.matrix array;
 	leaves : leaf array;
-	alpha : Gsl_matrix.matrix;
+	
+	workspace : workspace;
+	my_generation : int;
+	
+	alpha : Gsl_matrix.matrix; (** actually a sub-matrix of workspace.data *)
 	mutable z : float;
 	mutable have_alpha : bool;
-	beta : Gsl_matrix.matrix;
-	mutable have_beta : bool
+	beta : Gsl_matrix.matrix;  (** actually a sub-matrix of workspace.data *)
+	mutable have_beta : bool;
 }
 
-type workspace = Gsl_matrix.matrix
 let new_workspace tree dim =
 	let rows = 2 * (T.size tree) - (T.leaves tree)
-	Gsl_matrix.create rows dim
+	{ generation = min_int; data = Gsl_matrix.create rows dim }
 
 let empty = [||]
 let prepare ?workspace tree pms prior leaves =
@@ -44,14 +52,17 @@ let prepare ?workspace tree pms prior leaves =
 	if Array.length pms < n-1 then invalid_arg "CamlPaml.Infer.prepare: not enough P matrices"
 	
 	let workspace = match workspace with Some x -> x | None -> new_workspace tree k
-	let rows,cols = Gsl_matrix.dims workspace
+	workspace.generation <- (if workspace.generation = max_int then min_int else workspace.generation+1)
+	
+	let rows,cols = Gsl_matrix.dims workspace.data
 	if rows < (2*n-nl) || cols <> k then invalid_arg "CamlPaml.Infer.prepare: inappropriate workspace dimensions"
-	let alpha = Bigarray.Array2.sub_left workspace 0 (n-nl)
-	let beta = Bigarray.Array2.sub_left workspace (n-nl) n
+	let alpha = Bigarray.Array2.sub_left workspace.data 0 (n-nl)
+	let beta = Bigarray.Array2.sub_left workspace.data (n-nl) n
 	
 	for a = 0 to k-1 do beta.{n-1,a} <- prior.(a)
 	
-	{ tree = tree; pms = pms; leaves = leaves; alpha = alpha; z = nan; have_alpha = false; beta = beta; have_beta = false }
+	{ tree = tree; pms = pms; leaves = leaves; workspace = workspace; my_generation = workspace.generation;
+		alpha = alpha; z = nan; have_alpha = false; beta = beta; have_beta = false }
 
 (* Inside algo (aka Felsenstein algo.) *)
 let alpha_get x br =
@@ -106,10 +117,12 @@ let ensure_beta x =
 		x.have_beta <- true
 		
 let likelihood x =
+	if x.workspace.generation <> x.my_generation then failwith "CamlPaml.PhyloLik.likelihood: invalidated workspace"
 	ensure_alpha x
 	x.z
 
 let node_posterior inferred i =
+	if inferred.workspace.generation <> inferred.my_generation then failwith "CamlPaml.PhyloLik.node_posterior: invalidated workspace"
 	ensure_beta inferred
 	let k = snd (Gsl_matrix.dims inferred.alpha)
 	if inferred.z = 0. then
@@ -123,6 +136,7 @@ let node_posterior inferred i =
 			Array.init k (fun x -> (alpha_get inferred i).{x} *. inferred.beta.{i,x} /. inferred.z)
 
 let add_branch_posteriors ?(weight=1.0) inferred branch ecounts =
+	if inferred.workspace.generation <> inferred.my_generation then failwith "CamlPaml.PhyloLik.add_branch_posterior: invalidated workspace"
 	ensure_beta inferred
 	let k = snd (Gsl_matrix.dims inferred.alpha)
 	
diff --git a/lib/CamlPaml/Infer.mli b/lib/CamlPaml/PhyloLik.mli
similarity index 74%
rename from lib/CamlPaml/Infer.mli
rename to lib/CamlPaml/PhyloLik.mli
index 617e229..88e62a5 100644
--- a/lib/CamlPaml/Infer.mli
+++ b/lib/CamlPaml/PhyloLik.mli
@@ -1,29 +1,31 @@
-(** Probabilistic inference on phylogenetic trees given the substitution (P) matrices on each branch. These are low-level routines that should usually be used through the higher-level {{:PhyloModel}[PhyloModel]} abstractions. *)
+(** core phylogenetic likelihood calculations
 
-type intermediate
+These should usually be accessed through the higher-level {{:PhyloModel}[PhyloModel]} abstractions. *)
 
 (** Specifying a leaf (extant) character. Usually the extant character is known with certainty; in this case, use [`Certain] with the index of the character, e.g. [`Certain (Code.Codon61.index ('A','T','G'))]. Alternatively, you can specify an arbitrary probability distribution over extant characters. Lastly, you can specify to marginalize a leaf out of the likelihood calculations entirely. *)
 type leaf = [`Certain of int | `Distribution of float array	| `Marginalize]
 
 type workspace
 
-(** The calculations use a workspace of [((2 * T.size tree - T.leaves tree) * k)] [float]s where [k] is the alphabet size. As a performance optimization, you can create a workspace with [new_workspace tree k] and use it across multiple calls to [prepare]; otherwise it will allocate by itself. *)
+(** The calculations use a workspace of [((2 * T.size tree - T.leaves tree) * k)] [float]s where [k] is the alphabet size. As a performance optimization, you can create a workspace with [new_workspace tree k] and use it across multiple calls to [prepare]; otherwise, it will allocate automatically. *)
 val new_workspace : T.t -> int -> workspace
 
-(** [prepare tree p_matrices root_prior leaves] infers ancestral states, given:
+type intermediate
+
+(** [prepare tree p_matrices root_prior leaves] initializes the likelihood calculations, given:
 - [tree] the phylogenetic tree
-- [p_matrices.(i)] is the substitution matrix for the branch leading TO node [i] FROM its parent.
+- [p_matrices.(i)] is the substitution matrix for the branch leading {e to} node [i] {e from} its parent.
 - [root_prior] the prior probability distribution over characters at the root.
 - [leaves] is an array of [leaf]s (see above), the appropriate number for the tree
-- [workspace] is an appropriately sized workspace; it will be allocated if not given.
-@return an abstract value from which various information about ancestral states can be extracted (see below). If using a shared workspace, be sure to get all the results you need before the next call to [prepare].
+- [workspace] is an appropriately sized workspace; one will be allocated if not given.
+@return an abstract value from which various information about ancestral states can be extracted (see below). The time-consuming calculations are not actually performed until needed. If using a shared workspace, be sure to get all the results you need before the next call to [prepare].
 *)
 val prepare : ?workspace:workspace -> T.t -> P.matrix array -> float array -> leaf array -> intermediate
 
 (** calculate the probability of the leaves under the substitution model *)
 val likelihood : intermediate -> float
 
-(** compute the posterior probability distribution over characters at the specified node *)
+(** compute the posterior probability distribution over characters at the specified node. *)
 val node_posterior : intermediate -> int -> float array
 
 (** [branch_posteriors intermediate k] computes the posterior probability of each possible substitution on the specified branch [k]. That is, entry [(i,j)] of the returned matrix is [P(parent(k) = i && k = j | Leaves)] *)
diff --git a/lib/CamlPaml/PhyloModel.ml b/lib/CamlPaml/PhyloModel.ml
index f33352c..71cb73b 100644
--- a/lib/CamlPaml/PhyloModel.ml
+++ b/lib/CamlPaml/PhyloModel.ml
@@ -29,7 +29,7 @@ let q { qms = qms } br = qms.(br)
 let p { pms = pms } br = pms.(br) (* Array.map Array.copy pms.(br) *)
 let prior { prior = rp } = Array.copy rp
 
-let infer ?workspace m leaves = Infer.prepare ?workspace m.tree m.pms m.prior leaves
+let prepare_lik ?workspace m leaves = PhyloLik.prepare ?workspace m.tree m.pms m.prior leaves
 
 let checksum = 1., 1e-6
 
diff --git a/lib/CamlPaml/PhyloModel.mli b/lib/CamlPaml/PhyloModel.mli
index 35f0f2f..f9f547b 100644
--- a/lib/CamlPaml/PhyloModel.mli
+++ b/lib/CamlPaml/PhyloModel.mli
@@ -25,13 +25,13 @@ val prior : t -> float array
 *)
 val simulate : t -> (?a:(int array) -> unit -> int array)
 
-(** Probabilistically infer ancestral characters based on the given configuration of extant characters (leaves).
+(** Prepare likelihood calculations for the given configuration of extant characters (leaves).
 
-@param workspace (optional) a preallocated workspace for [Infer.prepare]
+@param workspace (optional) a preallocated workspace for {{:PhyloLik}PhyloLik.prepare}
 
-@return  {{:Infer}intermediate values}, from which additional information can be extracted.
+@return  {{:PhyloLik}PhyloLik.intermediate} values, from which additional information can be extracted.
 *)
-val infer : ?workspace:Infer.workspace -> t -> Infer.leaf array -> Infer.intermediate
+val prepare_lik : ?workspace:PhyloLik.workspace -> t -> PhyloLik.leaf array -> PhyloLik.intermediate
 
 
 (** {1 Symbolic parameterizations}
diff --git a/src/PhyloCSFModel.ml b/src/PhyloCSFModel.ml
index 968c5db..599d455 100644
--- a/src/PhyloCSFModel.ml
+++ b/src/PhyloCSFModel.ml
@@ -52,11 +52,11 @@ let lpr_leaves inst leaves t =
 	let ts = PM.P14n.tree_settings inst
 	ts.(0) <- t
 	let inst = PM.P14n.update ~tree_settings:ts inst
-	let workspace = Infer.new_workspace (PM.tree (PM.P14n.model inst)) Codon.dim
+	let workspace = PhyloLik.new_workspace (PM.tree (PM.P14n.model inst)) Codon.dim
 	let lik = ref 0.
 	leaves |> Array.iter
 		fun lvs ->
-			let lvslik = Infer.likelihood (PM.infer ~workspace:workspace (PM.P14n.model inst) lvs)
+			let lvslik = PhyloLik.likelihood (PM.prepare_lik ~workspace:workspace (PM.P14n.model inst) lvs)
 			lik := !lik +. log lvslik
 	!lik, inst