Non-tree patterns

[toivoh]

When the same pattern variable is repeated within a signature, e.g.

@pattern egal(x, x) = true
@pattern egal(x, y) = false

the pattern should require all occurrences to match the same value.

This should probably be accomplished by joining all the bound nodes into the same egal-equivalence class. This will require

• Pattern simplification driven by equivalence classes: alternate merging and simplification until reaching a fixed point.
• Nodes that have more than one argument node; at least for equivalence classes.
• Pattern printing:
  • support for DAG structures
  • name unnamed nodes with multiple references