function gamma1pm1( x ) {
	if ( isnan( x ) ) {
		return NaN;
	}
	if ( x < 0.0 ) {
		if ( x < -0.5 ) {
			// Best method is simply to subtract 1 from gamma:
			return gamma( 1.0+x ) - 1.0;
		}
		// Use expm1 on the logarithm of gamma:
		return expm1( -log1p( x ) + lgammaSmallImp( x+2.0, x+1.0, x ) );
	}
	if ( x < 2.0 ) {
		// Use expm1 on the logarithm of gamma:
		return expm1( lgammaSmallImp( x+1.0, x, x-1.0 ) );
	}
	// Best method is simply to subtract 1 from gamma:
	return gamma( 1.0+x ) - 1.0;
}
Computes `gamma(x+1) - 1`.

@param {number} x - input value
@returns {number} function value

@example
var v = gamma1pm1( 0.2 );
// returns ~-0.082

@example
var v = gamma1pm1( -9.2 );
// returns ~-1.0

@example
var v = gamma1pm1( 0.0 );
// returns 0.0

@example
var v = gamma1pm1( -3.0 );
// returns NaN

@example
var v = gamma1pm1( NaN );
// returns NaN