Perform completion on Lisp symbol preceding point.
Compare that symbol against the known Lisp symbols.
If no characters can be completed, display a list of possible completions.
Repeating the command at that point scrolls the list.
When called from a program, optional arg PREDICATE is a predicate
determining which symbols are considered, e.g. `commandp'.
If PREDICATE is nil, the context determines which symbols are
considered. If the symbol starts just after an open-parenthesis, only
symbols with function definitions are considered. Otherwise, all
symbols with function definitions, values or properties are considered.