Generates all ways to group elements with an operator of given arity.
Groupings[n, k]
Groupings[{a1, …, an}, k]
Groupings[{{a1, a2, …}, {b1, b2, …}, …}, k]
Groupings[aspec, f -> k]
Groupings[aspec, {f1 -> k1, f2 -> k2, …}]
Groupings[aspec, {{f1 -> k1, m1}, {f2 -> k2, m2}, …}]
Groupings[aspec, kspec, h]
Groupings[1, 2]
→ {1}