function mean( a, b, c ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		isnan( c ) ||
		!( a <= c && c <= b )
	) {
		return NaN;
	}
	return ( a + b + c ) / 3.0;
}
Returns the expected value of a triangular distribution.

@param {number} a - minimum support
@param {number} b - maximum support
@param {number} c - mode
@returns {number} expected value

@example
var v = mean( 0.0, 1.0, 0.5 );
// returns ~0.5

@example
var v = mean( 4.0, 12.0, 9.0 );
// returns ~8.333

@example
var v = mean( -4.0, 4.0, -1.0 );
// returns ~-0.333

@example
var v = mean( 1.0, -0.1, 0.5 );
// returns NaN

@example
var v = mean( 0.0, 1.0, 2.0 );
// returns NaN

@example
var v = mean( NaN, 4.0, 2.0 );
// returns NaN

@example
var v = mean( 0.0, NaN, 2.0 );
// returns NaN

@example
var v = mean( 0.0, 4.0, NaN );
// returns NaN