Two questions about the stackengine package:

Question

Borrowing from this post, I constructed the following example:

\documentclass{article}
\usepackage{stackengine}
\begin{document}
\[
  \setstackgap{L}{.7\baselineskip}
  \setstackgap{S}{9\baselineskip}
  \Vectorstack{H_1 > < H_0}
\]

\[
  \setstackgap{L}{.7\baselineskip}
  \Vectorstack{\alpha > < H_0}
\]

\[
  \setstackgap{L}{.7\baselineskip}
  \Vectorstack{\alpha\\ > < H_0}
\]
\end{document}

which produces the output below

My first question is: why doesn't the command \setstackgap{S}{9\baselineskip} result in a huge gap between the items in the first display? As far as I can tell, I'm using setstackgap exactly as in the manual.

My second question is: why is H_1 treated differently from \alpha? I.e., when I simply replace H_ with \alpha to obtain the second display, in the output, there is no space between \alpha and >; to get a space I need in the third display to type \alpha\\. Why this difference?

Answer 1

A \Vectorstack is a Long-type stack, and so you will find that setting \setstackgap{L}{} will change the interline baselineskip of a \Vectorstack, whereas a \setstackgap{S}{} will have no effect on a \Vectorstack. (As a reminder, a long-stack sets the baselineskip between stacked rows, whereas a short-stack sets the size of the dead-zone [empty] gap between row data.)

As to why H_1 acts differently than \alpha, the reason is because the default line separator in stackengine is a space. When you type H_1 blah, the space after the 1 is recognized as a space and interpreted as a linebreak in a stack. However, \alpha blah will not recognize the space after \alpha as a space, but as the terminator of \alpha. That is not a behavior of stackengine per se, but of LaTeX.

The easiest way is to change the end-of-line separator of stackengine with, for example, \setstackEOL{\\}. Then, you would use H_1 \\ blah and \alpha \\ blah to create two rows. However, if you choose to retain the space as the stack EOL character, then \alpha{} blah could be used to cause the space before blah to be recognized.

The MWE below compares long and short stacks, as well as adding {} after \alpha...

\documentclass{article}
\usepackage{stackengine}
\stackMath
\begin{document}
\[
  \setstackgap{L}{1.7\baselineskip}
  \setstackgap{S}{3\baselineskip}
  \Vectorstack{H_1 > < H_0}\quad
  \Shortstack{H_1 > < H_0}
\]

\[
  \setstackgap{L}{1.7\baselineskip}
  \Vectorstack{\alpha > < H_0}\quad
  \Vectorstack{\alpha{} > < H_0}
\]
\end{document}