{"ast_node_type":"other","tokens":[{"end":51,"kind":"keyword","start":0,"text":"//"}]}
// which has corresponding FIRST = {$a:expr, c, d}.
// The First set is computed on submatchers like `$($a:expr b),* $(c)* d`,
// Likewise, `$($a:expr b),* $(c)+ d` has FIRST = {$a:expr, c}.