Let's understand what we've just done The first thing we did was to declare a procedure, but with a little difference: it takes a parameter, called “color”.

procedure Drop3(color) {
  Drop(color)
  Drop(color)
  Drop(color)
}

Parameters (those names between brackets) are special, since they are replaced by real values when we invoke them. For example, if they are invoked like this…

program {
  Drop3(Black)
}

…what is executed is:

Drop(Black)
Drop(Black)
Drop(Black)

And if they are invoked like this…

program {
  Drop3(Red)
}

…what is executed is

Drop(Red)
Drop(Red)
Drop(Red)

Note that every time “color” appears, it is replaced by the value written in the invoking.

Let's see what can be done so far: write a program which drops three green stones using the procedure Drop3.