char-overflow-unicode.dfy(5,7): Error: char addition might overflow
 Asserted expression: (0 <= c0 as int + c1 as int && c0 as int + c1 as int < 55296) || (57344 <= c0 as int + c1 as int && c0 as int + c1 as int < 1114112)

Dafny program verifier finished with 0 verified, 1 error
