function fullIGammaPrefix( a, z ) {
	var prefix;
	var alz;

	alz = a * ln( z );
	if ( z >= 1.0 ) {
		if ( ( alz < MAX_LN ) && ( -z > MIN_LN ) ) {
			prefix = pow( z, a ) * exp( -z );
		}
		else if ( a >= 1.0 ) {
			prefix = pow( z / exp(z/a), a );
		}
		else {
			prefix = exp( alz - z );
		}
	}
	else {
		/* eslint-disable no-lonely-if */
		if ( alz > MIN_LN ) {
			prefix = pow( z, a ) * exp( -z );
		}
		else if ( z/a < MAX_LN ) {
			prefix = pow( z / exp(z/a), a );
		} else {
			prefix = exp( alz - z );
		}
	}
	return prefix;
}
Calculates the power term prefix `(z^a)(e^-z)` used in the non-normalized incomplete gammas.

@private
@param {number} a - function parameter
@param {number} z - function parameter
@returns {number} power term prefix