Illustration of the forward interpolation. Interpolation is performed for comoving knots j (top line) on intervals between the inertial knots i (bottom line). Each inertial interval [ i,i + 1) contains zero or more comoving knots denoted by . Corresponding l.h.s. weights (solid) and r.h.s. weights (dotted) are shown by sloped lines. Constant extrapolation is performed with weights on the (− ∞,i_red) interval, and with weights on the [ i_blue, + ∞) interval, both drawn with gray color. We note that the comoving knot , which exactly matches the inertial knot i, lies on the [ i,i + 1) interval, in agreement with the right-openness of all intervals.