<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def etype(self) -> Tuple[str, str]:
        '''Returns the expression's type.'''
        if self._expr[0] in ['number', 'boolean']:
            return ('constant', str(type(self._expr[1])))
        elif self._expr[0] == 'pvar_expr':
            return ('pvar', self._expr[1][0])
        elif self._expr[0] == 'randomvar':
            return ('randomvar', self._expr[1][0])
        elif self._expr[0] in ['+', '-', '*', '/']:
            return ('arithmetic', self._expr[0])
        elif self._expr[0] in ['^', '&', '|', '~', '=>', '<=>']:
            return ('boolean', self._expr[0])
        elif self._expr[0] in ['>=', '<=', '<', '>', '==', '~=']:
            return ('relational', self._expr[0])
        elif self._expr[0] == 'func':
            return ('func', self._expr[1][0])
        elif self._expr[0] == 'sum':
            return ('aggregation', 'sum')
        elif self._expr[0] == 'prod':
            return ('aggregation', 'prod')
        elif self._expr[0] == 'avg':
            return ('aggregation', 'avg')
        elif self._expr[0] == 'max':
            return ('aggregation', 'maximum')
        elif self._expr[0] == 'min':
            return ('aggregation', 'minimum')
        elif self._expr[0] == 'forall':
            return ('aggregation', 'forall')
        elif self._expr[0] == 'exists':
            return ('aggregation', 'exists')
        elif self._expr[0] == 'if':
            return ('control', 'if')
        else:
            return ('UNKOWN', 'UNKOWN')