Begin a CASE evaluation block.
See also CASE and END_CASE.

 Begin_case expression
 BEGIN_CASE marks the beginning of a CASE evaluation block. Each case in the block is tested until a match is found for the given expression. The statements following the matched CASE are executed until the next CASE statement is encountered. The program then resumes execution at the statement following the END_CASE statement.

 Begin_case a! evaluate variable a
 Case 0,1
 ! a = 0 or a = 1
 Case 2
 ! a = 2
 ! default case