IsBoolean returns True if the argument is of a boolean type.
This means it has to be either True, False, or an expression involving
functions that return a boolean result, like
=, >, <, >=, <=, !=, And, Not, Or.
Examples:
In> IsBoolean(a)
Out> False;
In> IsBoolean(True)
Out> True;
In> IsBoolean(a And b)
Out> True;