[ is also used to group several expressions at least one of which is true. For example, the following notation says that if x squared equals 9, then x equals either 3 or -3:
_
|
| x = 3
x^2 = 9 => |
| x = -3
|_
A
curly brace is used in a similar way to indicate that all expressions are true. For example, a
system of linear equations would look like this:
_
/
| 239x + 56y = 0
<
| 1917x + 70y = 1991
\_
You can include more than two expressons and nest as many
layers as you need.