Composition is the act of taking a stack and a function, and producing the type signature of the resulting stack. It can also be equivalenty seen as the composition of two functions' execution.

1. Generate constraints from the stated type sigs of your functions.
2. Match the RHS of the LHF, and the LHS of the RHF, from right to left. If the two symbols are:
   • Base types: Fail if one is not a parent type of the other. Otherwise, do nothing.
   • Both function types: Recursively generate constraints, starting with {a} = {b} and expanding the functions using the rule {--} ==> {a -- a} until one constraint set successfully unifies.
   • One is a variardic type: Recursively generate constraints, starting with a = and epanding the constraint to capture more values on the other side until one constraint set successfully unifies.
   • Else: generate a constraint a = b and move on.
3. Return the result of the first unification that succeeds. If you unify a -- b1 b2 and c1 c2 -- d, where b1/c1 are unmatched portions and b2/c2 are matched ones, you get a c1 -- b1 d.