[semidet]is_assoc(+Assoc)True if Assoc is an association list. This predicate checks
that the structure is valid, elements are in order, and tree is balanced
to the extent guaranteed by AVL trees. I.e., branches of each subtree
differ in depth by at most 1. Does not validate that keys are
sufficiently instantiated to ensure the tree remains valid if a key is
further instantiated.