def let meth, &proc
      proc || raise(ArgumentError, 'block is missing')
      vars[meth] = proc
      define_method(meth) { @__ms__vars[meth] ||= self.instance_exec(&proc) }
    end
def let meth, &proc
      proc || raise(ArgumentError, 'block is missing')
      vars[meth] = proc
      define_method(meth) { @__ms__vars[meth] ||= self.instance_exec(&proc) }
    end
@example
   describe Math do
     let(:x) { 0.1 }
     let(:y) { 1.0 }

     test 'x vs y' do
       assert(x) < y
     end
   end