last call optimisation

Normally when procedure A calls procedures B, C, .., Z, the

environment of procedure A is only discarded when procedure Z

returns and procedure A itself terminates. Using last call

optimisation, A's environment is discarded as Z is called.

This allows arbitrarily deep nesting of procedure calls

without consuming memory to store useless environments.

A special case of this is tail recursion optimisation where

the last procedure called is the calling procedure itself.