function mean( c ) {
	var k;
	if (
		isnan( c ) ||
		c <= 0.0
	) {
		return NaN;
	}
	k = ln( 1.0 + c );
	return ( c - k ) / ( c * k );
}
Returns the expected value of a Bradford distribution.

@param {PositiveNumber} c - shape parameter
@returns {PositiveNumber} expected value

@example
var v = mean( 0.1 );
// returns ~0.492

@example
var v = mean( 0.5 );
// returns ~0.466

@example
var v = mean( 10.0 );
// returns ~0.317

@example
var v = mean( 0.0 );
// returns NaN

@example
var v = mean( -1.0 );
// returns NaN

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