=== Sample programming task ===

Change/extend the compiler of the While language as follows:

  * Grade 2: Change the definition of variable names (identifiers) so that they have to contain at least one digit.
  * Grade 3: Add the ''elsif'' keyword to the language.
  * Grade 4: Change the syntax of ''if'' instructions so that it allows multiple branches. Example:
<code>
if x < 10 then
   a := 0;
elsif x = 0 then
   a := 1;
else
   a := 2;
end
</code>
  * Grade 5: Emit error message if the condition of an ''elsif'' is not boolean.