Renamed collect to garbage_collect.
Removed simplification.
Big bad commit.
A more sensible implementation of Graph.map.
Removed substitutions that delete instances.
Graphs have no loops.
Added Graph.leaves
Removed Graph.join.
Added Graph.vertices.
Computed dependencies between dirt_parameters.
Merged pos_neg_tyscheme and pos_neg_ty_scheme.
Added test examples for effect system.