CONVC
vp->tval &= ~(STR|CONVC|CONVO); /* mark string invalid */
vp->tval &= ~(NUM|CONVC|CONVO);
vp->tval &= ~CONVC;
vp->tval |= CONVC;
if ((vp->tval & CONVC) != 0 ||
vp->tval &= ~CONVC;
((vp->tval & CONVC) != 0 && vp->fmt != *fmt)) {
vp->tval |= CONVC;