Return a sub-matrix for all points where the given dimension has this value.
The dimension itself is not included in the result.
Note: if dim doesn't occur (e.g. because this is the unit set) and value is nullValue,
this returns the unit set (not the empty set).
Return a sub-matrix for all points where the given dimension has this value. The dimension itself is not included in the result. Note: if dim doesn't occur (e.g. because this is the unit set) and value is nullValue, this returns the unit set (not the empty set).