f
a
x
f
b
f
c
x
f
a
f
b
x
f
b
f
a
x
f
d
f
c
x
f
d
f
a
x
f
a
f
c
x
f
c
f
a
x
2.2
f
a
f
b
x
f
a
x
f
a
x
f
c
x
f
a
f
c
x
f
c
f
a
x
f
a
x
f
b
f
c
x
f
d
f
c
x
f
d
f
a
x
f
a
f
b
x
f
b
f
a
x
f
a
f
c
x
f
a
x
f
d
f
c
x
f
a
x
true
f
d
f
c
x
f
d
f
a
x
true
Failed!
NaTT
certifiable-1.6