a := series(z, z=0); e := series(1, x=0)

(1)

a+e

   There are 11 exposed and 8 unexposed library operations named + 
      having 2 argument(s) but none was determined to be applicable. 
      Use HyperDoc Browse, or issue
                                )display op +
      to learn more about the available operations. Perhaps 
      package-calling the operation or using coercions on the arguments
      will allow you to apply the operation.

   Cannot find a definition or applicable library operation named + 
      with argument type(s) 
                                     Any
                                     Any

      Perhaps you should use "@" to indicate the required return type, 
      or "$" to specify which version of the function you need.