In all the subroutines of this class I do not check if they were called correctly because I do that before. In this case ```constructLine()``` is only called if ```getNUniqueNodes() == 2```, i.e. I *know* that there are two points with different IDs in that element and checking again seems like a waste of time. (This is the same for all other checks, i.e. when constructing a Tet I simply use the first four unique nodes I find, I do not care if there are more because the method is only called if ```getNUniqueNodes() == 4```).