r
o
t
a
t
i
o
n
0
π
/
2
π
-
π
/
2
r
e
f
l
e
c
t
i
o
n
y
=
0
y
=
x
x
=
0
y
=
-
x
a
n
g
l
e
t
2
.
9
2
d
i
h
e
d
r
a
l
t
r
a
n
s
f
o
r
m
a
t
i
o
n
:
(
x
,
y
)
↦
(
y
,
x
)
i
d
e
n
t
i
t
i
e
s
:
(
c
o
s
(
π
/
2
-
t
)
,
s
i
n
(
π
/
2
-
t
)
)
=
(
s
i
n
(
t
)
,
c
o
s
(
t
)
)