syntax_data: {"ast_node_type":"impl","tokens":[{"end":73,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:         // <P1, P2, P0>, and then look up which of the impl args refer to
context_before:         // up the item-args <X, Y, Z> with the args on the impl ADT,
context_after:         // parameters marked as pure.