Need some way of pre-load reader macro before execution

[Takashi Kato]

R7RS-large includes reader extension (uniformed vector literals), however, at this moment, we need to add #!read-macro=... to enable it. Plus, unlike R6RS, R7RS doesn't require the #! to be a line comment if it doesn't have any meaning. So we need some way of pre-load.

One option can be command line, however, this affects the entire execution including library loading. The other option is a magical comment. Say if a comment starts with ;#! then it would do the same as the #!. This leads to backward incompatibility.