Syntax	A loopStatement is:   loop     statementsAndDeclarations   end loop
Description	A loop statement causes the statements (and declarations) in it to be repeatedly executed. This continues until terminated by one of its enclosed exit statements (or by an enclosed return or result statement).
Example	Output on separate lines: Happy, Happy, Happy, etc. loop put "Happy" end loop
Example	Read words up to the word Stop. var word : string loop get word exit when word = "Stop" end loop
Details	A loop statement can contain more than one exit, or none at all (in which case it is an infinite loop). When the exit when is at the beginning of the loop, the loop works like Pascal's do while; when at the end, the loop works like Pascal's repeat until. Just preceding the statements and declarations, you are allowed to write an "invariant clause" of the form: invariant trueFalseExpn This clause is equivalent to: assert trueFalseExpn.