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When an array is created with a line such as

[1 2 3 (O’Leary)]

the square brackets play a more active role than is immediately evident. The left bracket is a POSTSCRIPT operator that leaves an object called a mark on the stack.
As the interpreter continues through the program line, it puts more objects on the stack until it encounters a right bracket, which is an operator that creates an array containing the stack contents back to the topmost mark. The mark is dropped from the stack and the array remains.