A user defined function is a function defined by an algebraic expression. Support for this feature is courtesy of the wonderful exprtk library developed by Arash Partow.
A user defined function has a name, parameters and an expression. Example expressions are things like x+y or
sin(x). More details of the sorts of expressions possible can
be found in the User Defined Functions section of
the manual.
The parameters are specified as part of the name, so a user defined
function adding x and y would be called useradd(a,y) and the
sin example might be called mysin(x). Functions with up to two
arguments can be wired on the canvas. User defined functions can call
other user defined functions, so specifiying more than 2 parameters
can be a useful thing to do.