SYMBOL_VALID
sym->flags &= ~SYMBOL_VALID;
.flags = SYMBOL_CONST|SYMBOL_VALID,
.flags = SYMBOL_CONST|SYMBOL_VALID,
sym->flags |= SYMBOL_VALID | SYMBOL_WRITE;
.flags = SYMBOL_CONST|SYMBOL_VALID,
if (sym->flags & SYMBOL_VALID)
sym->flags |= SYMBOL_VALID;
sym->flags &= ~SYMBOL_VALID;