Boolean operations are also legal. (POSIX `bc`

does NOT have
boolean operations). The result of all boolean operations are 0 and 1
(for false and true) as in relational expressions. The boolean
operators are:

`!expr`

- The result is 1 if expr is 0.
`expr && expr`

- The result is 1 if both expressions are non-zero.
`expr || expr`

- The result is 1 if either expression is non-zero.

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