Return common substring of all completions of STRING in COLLECTION.
Test each possible completion specified by COLLECTION
to see if it begins with STRING. The possible completions may be
strings or symbols. Symbols are converted to strings before testing,
see `symbol-name'.
All that match STRING are compared together; the longest initial sequence
common to all these matches is the return value.
If there is no match at all, the return value is nil.
For a unique match which is exact, the return value is t.
If COLLECTION is an alist, the keys (cars of elements) are the
possible completions. If an element is not a cons cell, then the
element itself is the possible completion.
If COLLECTION is a hash-table, all the keys that are strings or symbols
are the possible completions.
If COLLECTION is an obarray, the names of all symbols in the obarray
are the possible completions.
COLLECTION can also be a function to do the completion itself.
It receives three arguments: the values STRING, PREDICATE and nil.
Whatever it returns becomes the value of `try-completion'.
If optional third argument PREDICATE is non-nil,
it is used to test each possible match.
The match is a candidate only if PREDICATE returns non-nil.
The argument given to PREDICATE is the alist element
or the symbol from the obarray. If COLLECTION is a hash-table,
predicate is called with two arguments: the key and the value.
Additionally to this predicate, `completion-regexp-list'
is used to further constrain the set of candidates.
(fn STRING COLLECTION &optional PREDICATE)