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\providecommandSynopses, one of:
\providecommand{cmd}{defn}
\providecommand{cmd}[nargs]{defn}
\providecommand{cmd}[nargs][optargdefault]{defn}
\providecommand*{cmd}{defn}
\providecommand*{cmd}[nargs]{defn}
\providecommand*{cmd}[nargs][optargdefault]{defn}
Defines a command, as long as no command of this name already exists.
If no command of this name already exists then this has the same effect
as \newcommand. If a command of this name already exists then
this definition does nothing. This is particularly useful in a file
that may be loaded more than once, such as a style file.
See \newcommand & \renewcommand for the description of the arguments.
This example
\providecommand{\myaffiliation}{Saint Michael's College}
\providecommand{\myaffiliation}{Saint Michael's College}
From \myaffiliation.
outputs ‘From Saint Michael's College’. Unlike \newcommand,
the repeated use of \providecommand does not give an error.