Adjust the dependent variables to have the smallest possible dependencies on the independent variables (x,y).
When dep= is specified as list of functions or names, only those dependent variables with new dependencies are required.
Adjust the dependent variables to depend on all independent variables.
Any invalid adjustment to the dependencies will trigger an error. For example, xi has to depend at least on y.
Note: the second argument dep = [xi] specifies the dependent variable xi to have no dependencies (i.e. to be a constant). These four LHPDEs should be essentially the same (i.e. they have same LHPDOs).