Write a TCL proc to find the factorial of a number where the number is passed as an argument.

Sure, here is a simple TCL procedure to calculate the factorial of a number:

proc factorial {n} {
    if {$n == 0} {
        return 1
    } else {
        return [expr {$n * [factorial [expr {$n-1}]]}]
    }
}

Here's how it works:

1. The proc keyword is used to define a new procedure named factorial. This procedure takes one argument, n, which is the number we want to find the factorial of.

2. Inside the procedure, we use an if statement to check if n is equal to 0. If it is, we return 1, because the factorial of 0 is 1.

3. If n is not equal to 0, we use the expr command to calculate the factorial. We multiply n by the factorial of n-1, which we find by recursively calling the factorial procedure.

4. The result of this calculation is then returned as the result of the factorial procedure.

You can call this procedure with a number to find its factorial. For example, factorial 5 will return 120.