Environment (or similar) which replaces symbol with command

Question

See this for an example of how replacing * with cdot would work.
I want to do the same type of thing but only within an environment. For example:
newenvironment{starmultiply}
{
% Something here...
}
{
% Undo previous effect
}

Another good usage would be to replace < and > with langle and rangle.
I don't always want to do this throughout the document, so DeclareMathSymbol can't be used (it is preamble only). Also,
newenvironment{starmultiply}
{
mathcode`*="8000 %
{catcode`*=activegdef*{cdot}}
}
{
% Undo previous effect
}

Doesn't seem to work. I get "Undefined control sequence" errors. Is there any solution which isn't super gross?

egreg · Accepted Answer

You can simply change the math code of *. Doing gdef* is not recommended and wouldn't work like that anyway; there are better methods to do the definition only locally. See the extended code below. documentclass{article} usepackage{amsmath} newenvironment{starmultiply} {mathcode`*=cdotignorespaces} {ignorespacesafterend} begin{document} This is a standard multiplication $acdot b$ begin{starmultiply} This is a nonstandard multiplication $a*b$ end{starmultiply} end{document} For langle and rangle it's a bit more complicated: documentclass{article} usepackage{amsmath} newenvironment{starmultiply} {% mathcode`*=cdot mathcode`<="8000 mathcode`>="8000 begingrouplccode`~=`< lowercase{endgrouplet~}langle begingrouplccode`~=`> lowercase{endgrouplet~}rangle ignorespaces} {ignorespacesafterend} begin{document} This is a standard multiplication $langle acdot brangle$ begin{starmultiply} This is a nonstandard multiplication $$ end{starmultiply} end{document}

Environment (or similar) which replaces symbol with command

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