\DeclareTextCompositeCommand ¶Synopsis:
\DeclareTextCompositeCommand{\cmd}{encoding}{arg}{code}
A more general version of \DeclareTextComposite that runs
arbitrary code with \cmd.
This allows accents on ‘i’ to act like accents on dotless i,
\i.
\DeclareTextCompositeCommand{\'}{OT1}{i}{\'\i}
See fontenc package, for a list of common encodings. Normally
\cmd will have already been declared with \DeclareTextAccent
or as a one argument \DeclareTextCommand.