The Concept of Action  31  Terms  are evaluated next and consist of a sequence of factors,  separated  by  multiplying  operators  (* ,  I,  di v,  mod,  and)  or  alternatively, simply a factor by itself.  Figure 4.e  Syntax diagram for  UnsignedConstant  Simple expressions  are  evaluated after terms  and consist of a  sequence  of terms,  separated  by  adding  operators  (+,  -,or) or  alternatively,  simply  a term  by  itself.  An  optional  sign-inversion  operator  (+, -)  may prefix the first term of a simple expression.  Figure 4.f  Syntax diagram for  Term  Finally  expressions  are  evaluated.  These  comprise a  simple  expression, a relational operator (=, <>, <=, >=,  >, in) and another  simple expression, or simply a simple expression itself.  Figure 4.g  Syntax diagram for  SimpleExpression  --.......,----------1 Constantldentifier  ~-------~--- I'--------~  UnsignedNumber  1---------1  nil  ~---------I  '---------1  CharacterString  1------------' 