function logcdf( x, a, b ) {
	return addon( x, a, b );
}
Evaluates the natural logarithm of the cumulative density function (CDF) for a uniform distribution with minimum support `a` and maximum support `b` at a value `x`.

@private
@param {number} x - input value
@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {number} evaluated logCDF

@example
var y = logcdf( 0.5, 1.0, 1.0 );
// returns ~-0.693

@example
var y = logcdf( 0.5, 2.0, 4.0 );
// returns ~-0.38

@example
var y = logcdf( 0.2, 2.0, 2.0 );
// returns ~-2.546

@example
var y = logcdf( 0.8, 4.0, 4.0 );
// returns ~-0.13

@example
var y = logcdf( -0.5, 4.0, 2.0 );
// returns -Infinity

@example
var y = logcdf( 1.5, 4.0, 2.0 );
// returns 0.0

@example
var y = logcdf( 2.0, -1.0, 0.5 );
// returns NaN

@example
var y = logcdf( 2.0, 0.5, -1.0 );
// returns NaN

@example
var y = logcdf( NaN, 1.0, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, 1.0, NaN );
// returns NaN