assert <test-expr> [<msg-expr>]
Check that a boolean expression is true for each row.
If the expression
<test-expr> does not
evaluate true for any row of the table, execution terminates
with an error.
As long as no error occurs, the output table is identical
to the input one.
<msg-expr> parameter is supplied,
then on failure it will be evaluated and its value presented
in the error message.
The exception generated by an assertion violation is of class
although that is not usually obvious if you are running from
the shell in the usual way.
Syntax for the
<msg-expr> arguments is described in the manual.