function validate( alpha, beta ) {
	if ( !isPositive( alpha ) ) {
		return new TypeError( format( 'invalid argument. First argument must be a positive number. Value: `%s`.', alpha ) );
	}
	if ( !isPositive( beta ) ) {
		return new TypeError( format( 'invalid argument. Second argument must be a positive number. Value: `%s`.', beta ) );
	}
	return null;
}
Validates parameters.

@private
@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {(Error|null)} error or null

@example
var err = validate( 1.0, 2.0 );
if ( err ) {
    throw err;
}