A minimal example: --------------- cut -------------------- \def\2#1#2{\vcenter{\halign{\hfil ##\hfil\cr #1\cr#2\cr}}} \def\ptwo#1#2{\left(\2{#1}{#2}\right)} \starttext \startformula \underbrace{\ptwo{specific}{heat}}_{1} \stopformula \stoptext --------------- cut -------------------- It works with 2013.05.28 00:36 MKIV current, correctly producing ( specific ) heat (with bigger parens) with an underbrace and a 1 underneath the brace. With 2013.12.26 12:50 MKIV beta, I get this error: ! Illegal parameter number in definition of \m_math_stackers_text_middle. system > tex > error on line 7 in file /home/sanjoy/sfse/test3.tex: Illegal parameter number in definition of ... 1 \def\2#1#2{\vcenter{\halign{\hfil ##\hfil\cr #1\cr#2\cr}}} 2 \def\ptwo#1#2{\left(\2{#1}{#2}\right)} 3 4 \starttext 5 6 \startformula 7 >> \underbrace{\ptwo{specific}{heat}}_{1} 8 \stopformula 9 10 \stoptext 11 <to be read again> \hfil \2#1#2l-\vcenter {\halign {\hfil ##\hfil \cr #1\cr #2\cr }} \ptwo #1#2l-\left (\2{#1}{#2} \right ) \math_stackers_make_double ...kers_text_middle {#6 }\edef \p_offset {\mathsta... l.7 \underbrace{\ptwo{specific}{heat}} _{1} I've attached the log file. The "\ptwo{specific}{heat}" by itself is fine. Only when it is wrapped in the \underbrace does the problem occur. -Sanjoy