Q:

adding equations gradually to beamer highlight (hf-tikz)

I am trying to highlight equations of a linear program starting from buttom and add constraints to the highlight. Here is my MWE:
\documentclass{beamer}
\usepackage{lmodern,amsmath,amssymb}
\usepackage[beamer]{hf-tikz}

\usetheme{Copenhagen}

\begin{document}

\section{Mysection}
\subsection{A subsection}
\begin{frame}{The equations}
\begin{block}{}
\[
  \begin{array}{r@{}r@{}r@{}l}
    \text{Max} \quad z=x_1 &{} + 12x_2 \\[\jot]
    \text{s.t.}\qquad 3x_1  &{} + \phantom{12}x_2 &{} &{} \leq 5 \\
    x_1 &         &{}  &{} \leq 16 \\
    \tikzmarkin<2->{a}
                    15x_1 &{} + \phantom{12}x_2 &           &{} = 14 \\ \tikzmarkend<2->{a} 
                    \tikzmarkin<1>{a}x_1 &{} &{}   &{} \geq 0 \\
                    &{} x_2 & &{} \geq 0\tikzmarkend{a}\\ 
  \end{array}
\]
\end{block}
\end{frame}

\end{document}

I get a weird 2-> expression in my slides and also my highlight box doesn't cover the equation fully (in second slide). Am I missing something? Thank you in advance for your help. 
NOTE: ht-tikz always needs two comilation. The manual says: Two compilation runs are always necessary: the first one to compute the position of the markers (also called delimiters) and the second one to place the box. So, if you get an error in the first run, don't panic! Run it again...

A:

Only \tikzmarkin receives the overlay specification (not \tikzmarkend); for the other issue you can use a \phantom; since the ending mark is the same for both frames, you can use it only once:
\documentclass{beamer}
\usepackage{lmodern,amsmath,amssymb}
\usepackage[beamer]{hf-tikz}

\usetheme{Copenhagen}

\begin{document}

\section{Mysection}
\subsection{A subsection}

\begin{frame}{The equations}
\begin{block}{}
\[
\begin{array}{r@{}r@{}r@{}l}
  \text{Max} \quad z=x_1 &{} + 12x_2 \\[\jot]
  \text{s.t.}\qquad 3x_1  &{} + \phantom{12}x_2 &{} &{} \leq 5 \\
  x_1 & & &{} \leq 16 \\
  \tikzmarkin<2->{a}15x_1 & {} + \phantom{12}x_2 & & {} = 14 \\ 
  \tikzmarkin<1>{a}x_1 & & & {} \geq 0 \\
  &{} x_2 & &{} \geq 0\phantom{5}\tikzmarkend{a}\\ 
\end{array}
\]
\end{block}
\end{frame}

\end{document}

For some reason the first run of the above code produces the error
! Package tikz Error: Cannot parse this coordinate.

The second run produces the desired output:

