zodiac:pexpand

• (zodiac:pexpand pattern env keywords) -> mixed
      pattern : zodiac:pat
      env : zodiac:penv
      keywords : (listof keyword)

  pexpand expands patterns in the context of a pattern environment and a list of keywords. The first argument is recursively copied verbatim into the output unless an identifier is encountered that is bound in the pattern environment and is not in the keyword list; this identifier is replaced by its binding, which has type read, in the pattern environment, and transcription proceeds accordingly. The output of compositing pexpand with structurize-syntax yields an object of type read, which can be subjected to further pattern matching, etc.