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