def wedge(otype) # =:= tensor
      if superior?(otype)
        self
      elsif otype.respond_to?(:superior?) && otype.superior?(self)
        otype
      else
        raise "wedge: unknown pair (#{self}) .wedge (#{otype})"
      end
    end
def wedge(otype) # =:= tensor
      if superior?(otype)
        self
      elsif otype.respond_to?(:superior?) && otype.superior?(self)
        otype
      else
        raise "wedge: unknown pair (#{self}) .wedge (#{otype})"
      end
    end
Needed in the type conversion of MatrixAlgebra