\edefappend

\edefappend is a way of adding on to an existing definition. It takes two arguments: the first is the control sequence name, the second the new tokens to append to the definition. The second argument is fully expanded (in the \edef that redefines the control sequence).

For example:

\def\foo{abc}
\def\bar{xyz}
\edefappend\foo{\bar karl}

results in \foo being defined as `abcxyzkarl'.