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

@private
@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} s - scale parameter
@param {number} m - location parameter
@returns {(Error|null)} error or null

@example
var err = validate( 1.0, 2.0, 1.3 );
if ( err ) {
    throw err;
}