Functions that may override the normal insertion of a completion choice.
These functions are called in order with four arguments:
CHOICE - the string to insert in the buffer,
BUFFER - the buffer in which the choice should be inserted,
MINI-P - non-nil if BUFFER is a minibuffer, and
BASE-SIZE - the number of characters in BUFFER before
the string being completed.
If a function in the list returns non-nil, that function is supposed
to have inserted the CHOICE in the BUFFER, and possibly exited
the minibuffer; no further functions will be called.
If all functions in the list return nil, that means to use
the default method of inserting the completion in BUFFER.