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Smallmatrix from AMS-TeX is not implemented in ConTeXt (Tex Live 2015), though crossed out in m-newmat.tex.

You can create your own matrix environment for inline use:

\definemathmatrix [smallmatrix] [left=\Big(, right=\Big), style=\scriptstyle, strut=0.7, distance=.25em]

\starttext

This is a example \m{ \startsmallmatrix \NC a \NC b \NR \NC c \NC d \NR \stopsmallmatrix } for a smallmatrix environment.

It looks pretty much like the AMS-LaTeX variation. Below is another example of use, as a displayed formula. ---- % Defines a small two-by-two matrix. \def\twomatrix#1#2#3#4{\startsmallmatrix\NC{#1}\NC{#2}\NR\NC{#3}\NC{#4}\NR\stopsmallmatrix} % The Pauli spin matrices, explicit forms. \def\pspint{{\twomatrix1001}} \def\pspinx{{\twomatrix0110}} \def\pspiny{{\twomatrix0{-𝕚}𝕚0}} \def\pspinz{{\twomatrix100{-1}}} Pauli spin matrices \startformula σ^0 ≔ \pspint, \quad σ^3 ≔ \pspinz, \quad σ^1 ≔ \pspinx, \quad σ^2 ≔ \pspiny \stopformula ----