Function is_in_follow 

fn is_in_follow(tok: &TokenTree, kind: NonterminalKind) -> IsInFollow

Returns true if frag can legally be followed by the token tok. For fragments that can consume an unbounded number of tokens, tok must be within a well-defined follow set. This is intended to guarantee future compatibility: for example, without this rule, if we expanded expr to include a new binary operator, we might break macros that were relying on that binary operator as a separator.