prolog-electric-colon-flag

prolog-electric-colon-flag is a variable defined in `prolog.el'.
Its value is

nil

Documentation:
Makes `:' electric (inserts `:-' on a new line).
If non-nil, pressing `:' at the end of a line that starts in
the first column (i.e., clause heads) inserts ` :-' and newline.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.