A number of MPE V/E-based systems use the MAIL facility to
direct the communication of information between processes.
This information transfer, however, is restricted to upward or
downward paths through the process tree structure, so that any
process can communicate only with its parent or children.
Between
any parent/child pair, only one such transfer is allowed at any
particular time.
The MAIL facility should not be confused with message files, which
can be used to transfer information between unrelated processes.
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