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It is also possible to use the calculated output value of the previous time step, by using "PREVIOUS_OUTPUT_VALUE" in uppercase letters in the expression (can be used multiple times). For each calculation time step "PREVIOUS_OUTPUT_VALUE" is replaced with the previous output value from the output time series. For the first calculation time step in the run period, the previous output value from before the start of the run period is read from the existing output time series in the database (e.g. from a previous run), if present. If for a given calculation time step the previous output value is not present or is unreliable, then "PREVIOUS_OUTPUT_VALUE" is replaced with NaN (missing value).
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