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The expression is always a comparison. The comparison operator is within XML one of (.ne., .eq., .gt., .ge., .lt., .le.). Each variable has to be a single word without spaces. Mathematical symbols or functions like e, pi or cos cannot be used as variableId, but they will be interpreted mathematically.
Note that in case of one of the variables of the expression contains missing values for a timestep, the expression fails, and no flags will be altered for this timestep.
Also manually edited flags will be left untouched.
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