The `patch` program is expected to be able to apply any patchfile syntaxes created by `diff`. Once upon a time, the default behavior of diff was to spit out an

The `patch` program is expected to be able to apply any patchfile syntaxes created by `diff`. Once upon a time, the default behavior of diff was to spit out an `ed` script (nowadays, it's behind the `-e` flag). So, to apply that syntax of patchfile, it invokes `ed`.