Test2

The math I want for mathjax to display: $$\begin{array}{} & \rightadj{\xrightarrow{\textstyle \rightcat f \rightadj K \leftadj Q \rightadj K}} \atop \rightadj{\xrightarrow[\phantom{\textstyle \rightcat f \rightadj K \leftadj Q \rightadj K}]{}} & X & \leftadj{\xtwoheadrightarrow{\textstyle \rightcat f \rightadj K \leftadj Q}}\\ \llap{\rightcat f \rightadj K \leftadj\eta} \leftadj{\Bigg\uparrow} \lower1.7ex\hbox{$\rightadj\parallel$} \rightadj{\Bigg\downarrow} \rlap{\rightcat f \rightadj\epsilon \rightadj K} && \Bigg\Vert && \rightadj{\Bigg\downarrow} \rlap{\rightcat f \rightadj\epsilon}\\ & \lower4pt\hbox{$\rightadj{\begin{smallmatrix} \xrightarrow{\Space{6em}{0ex}{0ex}} \\ \xrightarrow[\textstyle \rightcat f\rightadj K]{} \end{smallmatrix}}$} & X & \rightcat{\xrightarrow[\textstyle f]{}} & \\ \end{array}$$

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A simplified fragment of that math, not using the mathjax array construction: $$\xrightarrow{\textstyle f K Q K} \atop \xrightarrow{\textstyle f K Q K}$$

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The same, but wrapped in the array construction: $$\begin{array}{} \xrightarrow{\textstyle f K Q K} \atop \xrightarrow{\textstyle f K Q K} \end{array}$$

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Further experiment: $${a \atop b} \begin{array}{} {a \atop b} \end{array}$$