// TaggedChild(x,tag): returns the first child of node x tagged tag, or NULLT if there is none.\r
// Because of the balanced-parentheses representation of the tree, this operation is not supported\r
// efficiently, just iterating among the children of node x until finding the desired child.\r
// TaggedChild(x,tag): returns the first child of node x tagged tag, or NULLT if there is none.\r
// Because of the balanced-parentheses representation of the tree, this operation is not supported\r
// efficiently, just iterating among the children of node x until finding the desired child.\r