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To write a missing value (NaN) in the expression, you can use "0/0". Also note that an expression of "1/0" results in positive infinity, which is later replaced with NaN by the transformation module framework. However, "1/0" cannot be used in all cases to get an NaN value, since e.g. the expression "1 / (1/0)" gives 0 as a result, as expected.
Furthermore it is possible to use "if statements" in the expression. This can e.g. be used to get one output value if X is greater than 3 and get another output value if X is equal to or less than 3. For instance in the expression
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