def glpk_type(type)
      case type
      when :integer
        GLPK::GLP_IV
      when :binary
        GLPK::GLP_BV
      when :continuous
        GLPK::GLP_CV
      else
        fail type
      end
    end
def glpk_type(type)
      case type
      when :integer
        GLPK::GLP_IV
      when :binary
        GLPK::GLP_BV
      when :continuous
        GLPK::GLP_CV
      else
        fail type
      end
    end
Get the constant GLPK uses to represent our variable types