| ⊦ pos('6', '0' ' + '1' ' + '2' ' + '3' ' + '4' ' + '5' ' + '6' ' + '7' ' + '8' ' + '9' ') = 6 |
⊦ char ≠ char0 → pos(char, char0 ' + chars) = pos(char, chars) + 1
'6' ≠ '0' ⊦ pos('6', '1' ' + '2' ' + '3' ' + '4' ' + '5' ' + '6' ' + '7' ' + '8' ' + '9' ') + 1 = 6
Used terms: '1' ' + '2' ' + '3' ' + '4' ' + '5' ' + '6' ' + '7' ' + '8' ' + '9' ', '0', '6'