Ah, the multiply as a conditional. In older versions of Mathcad if the first operand of a multiply operator was zero, the second operand was not evaluated, the result was simply set to zero. This was removed in later versions, as it is actuallyh incorrect -- if the second operand of the multiply is an array, the result with a zero first operand should not be zero but rather an array of all zeros. IAC, such constructs need to be rewritten using if statements or if functions, or perhaps, depending on the usage, the on error construct.
If all uses of arg are of this form, I would simply redefine arg to allow for a zero argument.
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� � � � Tom Gutman