20240313
  Fix a fault with write commands when $auxfiles contains "d" and a
defined variable is split into linear and nonlinear parts.  In the
.col file, nonlinear parts are now indicated by a ".nl" suffix.
In the following silly example

	var x;  var y;
	var z = 4*y + y^4 - 6.38;
	var w = 8*x + abs(y) + 18.635;
	minimize O: 5*x + 8*z + 15;
	s.t. C: -200 <= 205*x -17*x^3 - 3.8*w <= 403.2869;
	solexpand;
	option auxfiles rd;
	write gfoo1;

the nonlinear part of z is y^4 and is denoted by z.nl, and the
nonlinear part of w is abs(y) and is denoted by w.nl in the solexpand
output and in foo1.col.