exist — Test for an argument
exist is a function that tests whether its argument is actually
a legitimate variable or function. If the argument exists, then exist
returns 1, or true if you prefer, and returns 0, or false, if the
argument doesn't exist.
Note that you can't pass a constant value as an argument, and passing
an argument that has enough brackets to look like a function, but
isn't really a function, gives an error message. This is caused by
not being able to evaluate the function.
In general, you should be using this function in general purpose
user-defined functions to make sure all the neccessary arguments are
present, and to provide defaults as appropriate.
Subsections