function validate( x0, gamma ) {
	if ( !isNumber( x0 ) || isnan( x0 ) ) {
		return new TypeError( format( 'invalid argument. First argument must be a number and not NaN. Value: `%s`.', x0 ) );
	}
	if ( !isPositive( gamma ) ) {
		return new TypeError( format( 'invalid argument. Second argument must be a positive number. Value: `%s`.', gamma ) );
	}
	return null;
}
Validates parameters.

@private
@param {number} x0 - location parameter
@param {PositiveNumber} gamma - scale parameter
@returns {(Error|null)} error or null

@example
var err = validate( 1.0, 2.5 );
if ( err ) {
    throw err;
}