theorem Th5: :: TURING_1:5

for x1, x2, x3 being Element of NAT holds
( Sum (Prefix (<*x1,x2,x3*>,1)) = x1 & Sum (Prefix (<*x1,x2,x3*>,2)) = x1 + x2 & Sum (Prefix (<*x1,x2,x3*>,3)) = (x1 + x2) + x3 )