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:

if x < 10 then
   a := 0;
elsif x = 0 then
   a := 1;
else
   a := 2;
end

• Grade 5: Emit error message if the condition of an elsif is not boolean.