Class dependency graph for Prefix expression evaluator.
An evaluation of a Prefixlist returns a Numberlist.

Example = <exps> List(Prefix).
Prefix : Number | Compound. ;multiple inheritance threat
  *SIG*
    *VIRTUAL* eval() *RETURNS* Number.
Compound = "(" Op <args> List(Prefix) ")".
  *SIG*
    eval() *RETURNS* Number
Op : Mulsym | Addsym 
  *sig* *virtual* apply_op(List(Number)) *returns* Number.
Mulsym = "*"
  *sig* apply_op(List(Number)) *returns* Number.
Addsym = "+"
  *sig* apply_op(List(Number)) *returns* Number.
List(S) ~ S {S}.

*instance* List(Prefix)
  *sig* eval() *returns* List(Number).
*instance* List(Number)
  *sig* 
    apply_times() *returns* Number,
    apply_plus() *returns* Number.
     
    
Prefix_list_it is an abbreviation for
Prefix_list_iterator

             Example	Prefix             Op
 	            \	^                  ^
	             \	|                  |
                      \ |                  |
		       V|                  |
Prefix_list_it<---------Prefix_list<-------Compound

                       
                        Number
			^
			|
			|
			|
Number_list_it<---------Number_list <----- Mulsym
                           ^
	                    \
	                     \
	                      \
	                       Addsym