open-paren-in-column-0-is-defun-start

open-paren-in-column-0-is-defun-start is a variable defined in `C source code'.
Its value is

t

Documentation:
Non-nil means an open paren in column 0 denotes the start of a defun.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.