Checks the specified expression for the available variables and alters the flags for the time steps where the expression succeeds. For instance, a check can compare the water flow before and after a river junction. It is required that all referenced variables in the expression have the same size. Either all variables are regular with the same timestep, or all variables are non-equidistant with the same times. The comparison can be for different parameters at one location or per location of a locationSet, or for one parameter at two locations or at one location and the locations of a location set, or between multiple equal location sets.