Turbo Pascal Statements 
                          While - Repetitive Statement 

    A While statement contains an expression that controls the repeated
    execution of a statement.  The statement can be a compound statement.
    The expression must evaluate to a standard boolean value.  Unlike
    the Repeat statement, the expression is evaluated before the statement
    is executed.  The statement will execute repeatedly, as long as the
    expression evaluates to TRUE.  If the expression evaluates to FALSE
    at the onset, the statement will never be executed.

                   +------------+             +-----------+
         While ---.| expression |--.  Do  ---.| statement |--.
                   +------------+             +-----------+


  EXAMPLE:  Var
               x, y : Integer;

            Begin
               ReadLn(y);
               while (y > 0) Do
                  Begin
                     Write('enter 0 to stop ');
                     ReadLn(y);
                  End;
            End.