Yacc help needed

[John Krist]

    I need some help.  I've just started using yacc and have been
  reading Stephen Johnson's brief report of it ("Yacc: Yet Another
  Compiler-Compiler").  In the back of the report, he gives three
  example yacc programs, including two calculator programs.

    I've keyed in both calculator programs, and while they both
  work fine with expressions such as "5+5", they signal an error
  when assignments are used ("a=5").

    I have verified that the programs are exactly entered as given
  in the book.  I would appreciate if someone could tell me what
  might be the problem.  I'm using BSD UNIX 4.3.

  Thanks,

  John Krist
  jk at astro.as.utexas.edu