Thanks that helps, but it doesn't quite do what I want. The assemble still fails if the blanked-out block has syntax errors in it. In C-like languages sometimes I like to comment out a large block of code that I'm having trouble getting to compile and slowly move the /* down to uncomment a little bit of it at a time so I'm not overwhelmed with the amount of code I have to get compiling all at once. Is there no way to do that with ca65?

c_comments: Allow C like comments using /* and */ as left and right comment terminators. Note that C comments may not be nested. There's also a pitfall when using C like comments: All statements must be terminated by "end-of-line". Using C like comments, it is possible to hide the newline, which results in error messages. See the following non working example:

lda #$00 /* This comment hides the newline
*/ sta $82

4.7 Conditional assembly
Please note that when using the conditional directives (.IF and friends), the input must consist of valid assembler tokens, even in .IF branches that are not assembled. The reason for this behaviour is that the assembler must still be able to detect the ending tokens (like .ENDIF), so conversion of the input stream into tokens still takes place. As a consequence conditional assembly directives may not be used to prevent normal text (used as a comment or similar) from being assembled.