A sample implementation of a macro that supports single-line comments where a comment expression is recognised at the beginning of a line and it transforms the entire content of that line into whitespace. Note that this does not allow a line comment to appear at the end of an expression!

While a regular expression could be specified to recognise whitespace as part of the comment marker a more sophisticated implementation would allow the comment marker to appear after an expression. It is also left as an exercise to implement multi-line comments (a la C /.../)