using diff() in a program

[cyrille de brébisson]

Hello

>in that case, I saw that I must use a very strange INPUT syntax {{var,[2]}} in someone else post.

INPUT is a home command, and in home, you can not use a variable that has not been defined.
so, INPUT is (normally) unable to let you enter an equation in 'x' if 'x' does not exits.
INPUT takes as first argument a variable and let you request it from the user.
but INPUT can also take 2 or more variables, using {var1,.. varn} syntax.
Furthermore, you can tell INPUT that the variable requested have to follow a given type or types. This is done by placing the variable in a list and having the allowed types as a matrix in the 2nd element of the list.
It is only to make it easier for the system to distinguish between the use of the 3 syntaxes that you are forced, if you want to specify the type for one single variable to still create a list of list...

anyhow, here the 2 means string. So it forces the input to be a string. and strings, contrary to expression can contain anything you want, including the text of undefined variables.

This is why you use {{var [2]}}

Cyrille