Definition.
Let
M
be an
L
-structure,
A
⊆
M
. Given an
L
A
-formula
φ
(
x
1
,
…
,
x
n
)
, define
[
φ
(
x
1
,
…
,
x
n
)
]
=
{
p
∈
S
n
M
(
A
)
:
φ
(
x
1
,
…
,
x
n
)
∈
p
}
.