code: def index(fks, id):
      
      for i, j in fks.items():
          if fks[i][1] == id:
              return i
      return None
docs: Look for the id on fks, fks is an array of arrays, each array has on [1]
      the id of the class in a dia diagram.  When not present returns None, else
      it returns the position of the class with id on fks