Solves a tridiagonal system for one of coordinates (x or y) of first Bezier control points. @param rhs Right hand side vector. @return Solution vector.
function(rhs) {
            var n = rhs.length;
            // `x` is a solution vector.
            var x = [];
            var tmp = [];
            var b = 2.0;
            
            x[0] = rhs[0] / b;
            // Decomposition and forward substitution.
            for (var i = 1; i < n; i++) { 
	        tmp[i] = 1 / b;
	        b = (i < n - 1 ? 4.0 : 3.5) - tmp[i];
	        x[i] = (rhs[i] - x[i - 1]) / b;
            }
            for (i = 1; i < n; i++) {
                // Backsubstitution.
	        x[n - i - 1] -= tmp[n - i] * x[n - i]; 
            }
            return x;
        }