[NTG-context] hairline in a matrix

Hi, I could not figure out how to add a hairline between two rows of a matrix. Following example is compiling but only results in a bigger vertical space. Infos/Hints are welcome... \startformula \startmatrix    \NC {a_1 } \NC {a_2 } \NC {a_3 }  \NC  \ldots      \NC {a_X }  \NR    \NC {b_1 } \NC 0      \NC 0       \NC 0            \NC 0       \NR    \NC 0      \NC {b_2 } \NC 0       \NC 0            \NC 0       \NR    \NC 0      \NC 0      \NC  \ddots \NC 0            \NC 0       \NR \NC[n=5] {\text{\hairline}} \NR    \NC 0      \NC 0      \NC 0       \NC {b_{X - 1} } \NC 0       \NR \stopmatrix \stopformula