[ 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.