So that other preprocessors are able to get a view of the source files as the Compiler would after the effect of REPLACE and REPLACING, the preprocessor returns additional lines as documented here. Note, however, that the Compiler still performs this REPLACING itself except as noted below. The additional lines are passed for information only.
If the preprocessor ever detects that a line will be modified, it will add 32 to the value of resp-main it returns. The line or lines thus marked will be followed by one or more lines marked with the value of 8 in resp-main, containing what would be in the line after it was modified. As these changes are for information only, other preprocessors receiving the lines should pass these values back out again for any other preprocessors in the stack; when the Compiler receives lines marked with values in the range 33 to 64 it subtracts 32, and it ignores lines marked with an 8.
If a preprocessor wishes to modify a line affected in this way, it should do so as normal, however it should mark the new line with a 9 rather than a 1 to inform the Compiler that this new, inserted, line does not need to be tested for the effect of REPLACE or REPLACING.