Currently a function returns a boolean for a state change.

If we change this to an integer and constants, this allows more expressive stepping.

• -n: step in a negative (left) direction.
• 0: Keep open, stay put; allowing a sink to continue sinking.
• [+]n: continue positive (right), similar to True.
• PASS: Assert the sequence as a match, resetting the path.
• DROP|FAIL: Assert the sequence as a fail or drop, resetting the path.