bool parse();	/* (line_p l, *l_out; offset nbytes;
			 * int    level; int    (*action0) ())
			 * This is a recursive descent parser for
			 * EM expressions.
			 * It tries to recognize EM code that loads exactly
			 * 'nbytes' bytes on the stack.
			 * 'l' is the last instruction of this code.
			 * On toplevel, when an expression has been
			 * recognized, the procedure-parameter
			 * 'action0' is called, with parameters: the first and
			 * last instruction of the expression and the number of
			 * bytes recognized.
			 */