Abstract: The present invention makes it possible for computer programmers to selectively examine the effects of preprocessing operations on computer source code. Where a preprocessor construct, such as a macro, appears in the source code, a user may selectively expand the construct to see the effects of expansion. If a macro expands into text which contains other macro calls, the user may selectively expand the other macro calls. The present invention allows the user to go back and forth between unexpanded, partly expanded, and fully expanded constructs in order to obtain a better understanding of the effect of macro expansion on the original source code. The present invention applies to any programming language which supports macros and/or other preprocessing constructs. It also applies to preprocessors that are independent of any programming language, such as the M4 preprocessor. The invention may also be included in another computer program, such as a debugger.