diff --git a/src/Tree/Optimizer.nqp b/src/Tree/Optimizer.nqp
index 3e59d61..66ddcdc 100644
--- a/src/Tree/Optimizer.nqp
+++ b/src/Tree/Optimizer.nqp
@@ -35,17 +35,8 @@ method find-pass ($name) {
 method register ($transformation, *%adverbs) {
     my $pass := Tree::Optimizer::Pass.new($transformation, |%adverbs);
     %!passes{$pass.name} := $pass;
-    my $depends-on := %adverbs<depends-on>;
-    if $depends-on {
-        my @dependencies;
-        if pir::isa__IPP($depends-on, String) {
-            @dependencies := pir::split__PSS(' ', $depends-on);
-        } else {
-            @dependencies := $depends-on;
-        }
-        for @dependencies -> $dependency {
-            self.add-dependency($pass.name, $dependency);
-        }
+    for $pass.dependencies -> $dependency {
+        self.add-dependency($pass.name, $dependency);
     }
 }
 
diff --git a/src/Tree/Optimizer/Pass.nqp b/src/Tree/Optimizer/Pass.nqp
index b160e30..9feb081 100644
--- a/src/Tree/Optimizer/Pass.nqp
+++ b/src/Tree/Optimizer/Pass.nqp
@@ -1,10 +1,14 @@
 class Tree::Optimizer::Pass;
 
+has @!dependencies;
 has $!name;
 has $!recursive;
 has $!transformation;
 has $!when;
 
+our multi method dependencies () { @!dependencies; }
+our multi method dependencies (@deps) { @!dependencies := @deps; }
+
 our multi method name () { $!name; }
 our multi method name ($name) { $!name := $name; }
 
@@ -29,11 +33,17 @@ my $current-gen-name := 0;
 sub gen-name () {
     '__unnamed_' ~ $current-gen-name++;
 }
-method BUILD (:$transformation, :$name, :$recursive, :$when, *%ignored) {
+method BUILD (:$transformation, :$name, :$recursive, :$when, *%rest) {
     $!name := $name || gen-name();
     $!recursive := $recursive || 0;
     $!transformation := $transformation;
     $!when := $when;
+    my $depends-on := %rest<depends-on>;
+    if $depends-on {
+        @!dependencies := (pir::isa__IPP($depends-on, String)
+                           ?? [ $depends-on ] 
+                           !! $depends-on);
+    }
 }
 
 method run ($tree) {