Variable: prolog-left-paren
prolog-left-paren is a variable defined in prolog.el.gz.
Value
"[[({]"
Documentation
The characters used as left parentheses for the indentation code.
Source Code
;; Defined in /usr/src/emacs/lisp/progmodes/prolog.el.gz
(defconst prolog-left-paren "[[({]" ;FIXME: Why not \\s(?
"The characters used as left parentheses for the indentation code.")