| str = char + str0 ⊦ str = "", substring(str, 0, m + 1) = char ' + substring(str0, 0, m) |
⊦ char ' + str0 = "", substring(char ' + str0, 0, m + 1) = char ' + substring(str0, 0, m)
Used simplifier rules: chartostring-01: ⊦ char + str = char ' + str; from specification string-append