# File lib/eymiha/units/units.rb, line 67
    def Units.find_by_derivation(derivation)
      matches = @@measures.values.uniq.select { |measure|
        if measure.derived
          measure == derivation
        elsif derivation.size == 1
          a = derivation.to_a[0]
          a[0] == measure and a[1] == 1
        else
          false
        end
      }
      case matches.size
      when 0 then nil
      when 1 then matches[0]
      else raise UnitsException,
                "Multiple UnitsMeasures with same derivation found"
      end
    end