http://www.tcl.tk/man/tcl8.6/TclCmd/transchan.htm 

This is a protocol for defining a '''channel transform''', which is a way to intercept the reading and writing of bytes on a channel with a Tcl script and modify them (e.g., handling encryption, logging or compression). The script is attached via [chan push].

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[[replace with details of this command]]

[[[DKF]: It's not a command; it's a protocol that a command prefix should adhere to to participate as a [chan push%|%channel transform].]]

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