denotational semantics

 - syntax
 -    a*b+c
 -    (a*b)+c    vs    a*(b+c)
 - assume code "parsed up" into nice tree
 -  natural way to write:
       EXPR(ap,+,EXPR(ap,*,EXPR(var,a),EXPR(var,b)),EXPR(var,c))
 - instead write a*b+c and assume "natural decomposition"

Valuation Function : maps code fragments to "meaning"

  type of "meaning" varies depending on nature of language

  need to talk about control flow, develop notion of "continuation"