With a recent ConTeXt, the following file (run with "mpost -mem=metafun test.mp") % use ConTeXt to typeset labels (needs -mem=metafun option) verbatimtex %&cont-en etex beginfig(100) % flow arrow label(btex $\displaystyle\buildrel \rm Flow \over {=\joinrel=\joinrel\Longrightarrow}$ etex, origin); endfig; end fails with: ! Unable to make mpx file. l.7 label(btex $\displaystyle\buildrel \rm Flow \over Transcript written on fig.log. The error in mpxerr.log is: ! Illegal unit of measure (pt inserted). <to be read again> = <argument> = \joinrel =\joinrel \Longrightarrow \buildrel ...mathrel {\mathop {\kern \zerocount #2 }\limits ^{#1}} l.14 {=\joinrel=\joinrel\Longrightarrow} $% It works with ConTeXt 2008.10.31. It fails with ConTeXt 2009.06.04 (with texlive2007 from Debian or with vanilla texlive2008).