| ⊦ 5 = pos('7', '2' ' + '3' ' + '4' ' + '5' ' + '6' ' + '7' ' + '8' ' + '9' ') |
⊦ char ≠ char0 → pos(char, char0 ' + chars) = pos(char, chars) + 1
'7' ≠ '2' ⊦ 5 = pos('7', '3' ' + '4' ' + '5' ' + '6' ' + '7' ' + '8' ' + '9' ') + 1
Used terms: '3' ' + '4' ' + '5' ' + '6' ' + '7' ' + '8' ' + '9' ', '2', '7'