Sequence 1,3,4,7,1,8,9,7, (). 3.9... What are the rules of this sequence

Sequence 1,3,4,7,1,8,9,7, (). 3.9... What are the rules of this sequence

1+3=4
3+4=7
4 + 7 = 11 bit 1
1+8=9
8+9=17
9+7=16
So fill in the brackets with 6