GEN
if2(GEN c1, GEN c2, GEN tt, GEN tf, GEN ft, GEN ff)
{
  if (gequal0(c1))
    if (gequal0(c2))
      return ff ? closure_evalgen(ff) : gnil;
    else
      return ft ? closure_evalgen(ft) : gnil;
  else
    if (gequal0(c2))
      return tf ? closure_evalgen(tf) : gnil;
    else
      return tt ? closure_evalgen(tt) : gnil;
}