Error: alias clause lists an operator for a feature such that
  it is not a query or
  its argument number is not 0 for unary operator or
  its argument number is not 1 for binary operator or
  class has other feature with the same alias name and argument number.
What to do: ensure that feature is a query and has no arguments for unary
  operator or has one argument for binary operator and class has no other
  feature with the same operator name and the same number of arguments.
