Co-processing functions

When writing programs to use as co-processes, the following tips may be useful. If the co-process program is written in C language, be sure to flush output after writing to the pipe. (Currently, awk(1) and sed(1) cannot be used in a co-process program because they do not flush after lines of output.) Shell scripts are well-mannered, but slow. C language is recommended. If possible, use the default send_string, rpath, and wpath. In most cases, expect_string has to be specified. This, of course, depends on the co-process.