diff --git a/pkg/R/feasibility.R b/pkg/R/feasibility.R
index 6061485..51ef8b2 100644
--- a/pkg/R/feasibility.R
+++ b/pkg/R/feasibility.R
@@ -2,7 +2,7 @@
 # find straigtforward contradictions of the form 0 <= b or 0 == b 
 has_contradiction <- function(A,b,tol){
   if (nrow(A)==0) return(FALSE)
-  if (ncol(A)==0 & abs(b) > tol) return(TRUE)
+  if (ncol(A)==0 & all(abs(b) > tol)) return(TRUE)
   any(rowSums(abs(A)>tol) == 0 & (abs(b)>tol))
 }
 
diff --git a/pkg/R/substval.R b/pkg/R/substval.R
index cf3aac4..2e84bd7 100644
--- a/pkg/R/substval.R
+++ b/pkg/R/substval.R
@@ -9,6 +9,7 @@
 #' @param b \code{[numeric]} vector 
 #' @param variables \code{[numeric|logical|character]} vector of column indices in \code{A}
 #' @param values \code{[numeric]} vecor of values to substitute.
+#' @param remove_columns Remove spurious columns when substituting?
 #'
 #' @return A \code{list} with the following components:
 #' 
@@ -19,13 +20,17 @@
 #'
 #'
 #' @export
-subst_value <- function(A, b, variables, values){
+subst_value <- function(A, b, variables, values, remove_columns=FALSE){
   check_sys(A=A, b=b)
   if ( is.character(variables) ){
     variables <- match(variables,colnames(A))
   }
 
   b <- as.vector(b - A[,variables,drop=FALSE] %*% values)
-  A <- A[,-variables,drop=FALSE]
+  if (remove_columns){
+    A <- A[,-variables,drop=FALSE]
+  } else {
+    A[,variables] <- 0
+  }
   list(A=A, b=b)
-}
\ No newline at end of file
+}
diff --git a/pkg/tests/testthat/test_substitute.R b/pkg/tests/testthat/test_substitute.R
index a9837b4..8cc347b 100644
--- a/pkg/tests/testthat/test_substitute.R
+++ b/pkg/tests/testthat/test_substitute.R
@@ -10,17 +10,17 @@ subst_value(A,b,1,1)
 test_that("value substitution",{
 
   expect_equivalent(
-    subst_value(A,b,1,0)
+    subst_value(A,b,1,0,remove_columns=TRUE)
     , list(A=A[,2:3],b=b)
   )
   expect_equivalent(
-    subst_value(A,b,c(1,3),c(0,1))
+    subst_value(A,b,c(1,3),c(0,1),remove_columns=TRUE)
     ,list(A=A[,2,drop=FALSE],b=b-9:12)
   )
   colnames(A) <- paste0("x",seq_len(ncol(A)))
   expect_equivalent(
-    subst_value(A,b,"x1",0)
+    subst_value(A,b,"x1",0,remove_columns=TRUE)
     , list(A=A[,2:3],b=b)
   )
   
-})
\ No newline at end of file
+})