[NTG-context] typescripts

I played with typescripts and have a problem. I can use the beramono font in this way: \definefont [Bera] [fvmr8a at 9.5pt] \starttext \Bera \input knuth \stoptext but how would I insert it into a typescript of libertine, where the mono font should used from bera? This did not work: ... \starttypescript[libertine] \definetypeface[libertine][rm][serif][libertine][default] \definetypeface[libertine][ss][sans] [libertine][default] \definetypeface[libertine][tt][mono] [file:fvmr8a.pfb] [default][rscale=0.85] \definetypeface[libertine][mm][math] [modern] [default] \stoptypescript ... Herbert