rCalc modification: using user-defined functions in rCalc 0.3.7

rCalc is a very good symbolic calculator, very simple to use and it can be very useful when doing many calculus, especially thanks to variables.
However, in two years of use, I felt the lack of a feature I really need: the ability to create my own functions.
For exemple, create a function f that does f(x) = 2*x*x + 7*cos(x) - abs(x)

Then, it would just give you the result for x=2 when typing f(2), for example, and it would speed up your work when you need to do several calculus with the same function.

So I made this quick (and I'm sure, dirty) patch that enables you to:
- create functions: func <function_name>(x)=<your_function>
   (example: func g(x)=2*x)

- list ALL the functions (that is, the ones you have created and the built-in ones): ls functions  (or just ls func)

- use the function: function_name(mathematical_expression)
   (example: g(cos(5*2 - 10))  or just g(1) )

- remove an existing function: rm function_name  (just like you remove variables)

- the functions are saved in ~/.rcalc/  and are automatically reloaded at startup

The functions are in fact a .c file compiled to a .so and dynamically linked to the executable, all this when the func command is called. Therefore:
- you need a working gcc installed
- the functions take a little time to compile when you use the func command (but it's usually less than 1 or 2 second)
- once the function is compiled, the call to the function and the calculus is very fast, since the code is compiled.
- syntax errors are automatically detected by compiler and/or linker, and prints message inside rCalc (see screenshot for the 2 error messages that are possible).

See this screenshot for these functions in action.
Download the patch. (this patch is against rcalc 0.3.7)

Apply it like this (for example) :
~/rcalc-0.3.7$ zcat ../rcalc-0.3.7-functions.patch.gz | patch -p1 -E

Clément Bourdarias, last modification: June, 19th 2002.