*! version 3.0.0  end-point rule utility
program define nlsm_epu /* s[n-2] s[n-1] s[n]  or s[3] s[2] s[1] */
	                /*    s2     s1    s0       s2    s1   s0 */
	local s2 = `1'
	local s1 = `2'			/* lazy 	*/
	local s0 = `3'			/* actual 	*/
	local p = `s1'+2*(`s1'-`s2')	
	#delimit ;
	mac def S_1=cond(
		`p'<=max(`s1',`s0') & `p'>=min(`s1',`s0'),`p',
		cond(
		`s1'<=max(`p',`s0') & `s1'>=min(`p',`s0'),`s1',
		`s0')) ;
	#delimit cr
end
exit

Equivalency of formulas.

End-point adjustment E is (Velleman 3.4)

	z_1 = med{ 3*z_2 - 2*z_3, y_1, z_2}

Equivalent to call:

	nlsm_epu z_3 z_2 y_1

Because:

	p = z_2+ 2*(z_2 - z_3) = 3*z_2 - 2*z_3