Dear Stata users,

I have a task in hand that seems to be accomplished using -stack-, but it failed. My data is like follows:
Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(a2013 a2014 a2015 b2013 b2014 b2015 c2013 c2014 c2015 d2013 d2014 d2015)
3.7 3.3 3.2 2.4 2.1   2 454.4 465.2 468.6 220.4 232.8 252.4
  7 7.2 7.5   3   3   3 571.4 584.7 589.3 275.3 285.5 311.1
2.9 2.9   3 2.7 2.5 2.8 316.3 318.6 320.8 148.2 151.5 164.3
1.9 1.8 1.9 2.5 2.2 2.3 291.3 294.1 294.9  99.8 101.3 109.3
 .9   1   1   2   2 2.2 148.2 149.8 150.7  75.1  77.2  82.8
end
And I want to stack variables a2013, a2014, and a2015 into varA, and variables b2013, b2014, and b2015 into varB, and so forth. I find that the official command -stack- cannot do this work.