15 Compatibility with other versions of m4

This chapter describes the differences between this implementation of m4, and the implementation found under UNIX, notably System V, Release 3.

There are also differences in BSD flavors of m4. No attempt is made to summarize these here.

• Extensions: Extensions in GNU M4
• Incompatibilities: Facilities in System V m4 not in GNU M4
• Other Incompatibilities: Other incompatibilities