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

@private
@param {PositiveInteger} k - shape parameter
@param {PositiveNumber} lambda - rate parameter
@returns {(Error|null)} error or null

@example
var err = validate( 1, 2.5 );
if ( err ) {
    throw err;
}