prederror, conditionals

For my ideas about a mathematical conditional, see
to_poly_solve_extra.lisp. A few examples:

   (%i1) load("to_poly_solver.mac")$

   (%i15) f(x) := %if( (x <3) %or (x >= 3), -1, 1)$

   (%i16) f(a);
   (%o16) -1

Compare this to 'if'

   (%i17) g(x) := if (x <3) or (x >= 3) then -1 else 1$
   (%i18) g(a);

   (%o18) if a<3 or a>=3 then -1 else 1
   (%i19) is(%);

   (%o19) unknown

Further,

  (%i7) sol : to_poly_solve(a * x = b, x, 'parameters = ['a,'b]);
  (%o7) %union(%if(a#0,[x=b/a],%union()),%if((a=0)  %and
  (b=0),[x=%c5],%union()))

  (%i8) sublis([a=0,b=1], sol);
  (%o8) %union()

  (%i9) sublis([a=0,b=0], sol);
  (%o9) %union([x=%c5])

  (%i10) sublis([a = 42], sol);
 (%o10) %union([x=b/42])

Barton