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From the Decker reference manual, Built-In Functions, footnote 6:

The behavior of random[x y] depends on the type of x and whether or not y is provided:

• if x is a number, treat it as if it were range x.
• if x is anything else, choose random elements from it.
• if y is missing, the result will be a single random element.
• if y is positive, choose a list of y random elements.
• if y is negative, choose a list of |y| random elements without repeats, provided sufficient elements in y.