- Do not reduce a variable if a resolver return a null value.   The variable has to be resolved at evaluation time. - A Variable_Mapper can return a null expression to indicate   that a variable does not exist (faster than handling an exception).
- Do not reduce a variable if a resolver return a null value.
  The variable has to be resolved at evaluation time.
- A Variable_Mapper can return a null expression to indicate
  that a variable does not exist (faster than handling an exception).


stcarrez/ada-el