-substitute replace expr = CoreSubst.substExpr subs expr
- where subs = foldl (\s (b, e) -> CoreSubst.extendSubst s b e) CoreSubst.emptySubst replace
+substitute [] expr = expr
+-- Apply one substitution on the expression, but also on any remaining
+-- substitutions. This seems to be the only way to handle substitutions like
+-- [(b, c), (a, b)]. This means we reuse a substitution, which is not allowed
+-- according to CoreSubst documentation (but it doesn't seem to be a problem).
+-- TODO: Find out how this works, exactly.
+substitute ((b, e):subss) expr = substitute subss' expr'
+ where
+ -- Create the Subst
+ subs = (CoreSubst.extendSubst CoreSubst.emptySubst b e)
+ -- Apply this substitution to the main expression
+ expr' = CoreSubst.substExpr subs expr
+ -- Apply this substitution on all the expressions in the remaining
+ -- substitutions
+ subss' = map (Arrow.second (CoreSubst.substExpr subs)) subss