harmony:proper_tail_calls [ES Wiki]: "We need to define when a call is in possible tail position for safety purposes, and proper tail position for liveness purposes. An implementation MAY implement a call in possible tail position as a proper tail call (PTC) and MUST implement a call in proper tail position as a PTC. (Loosely speaking, implementing a call as a PTC is also known as tail call optimization.) The old ES4 proposal for proper tail calls should be adapted to provide these definitions for ES-Harmony."
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