Ugly matrix spacing

[Vyatcheslav Yatskovsky <yatskovsky@gmail.com>]

Hi,
I'm ConTeXting a text that follows:

%%%%%%%%%%%%%%%%%
\setupwhitespace[big]
\setupinterlinespace[line=2.5ex]
\starttext

The Leslie Matrix is a matrix $L[1:X, 1:X]$ of the form

$$\left( {\matrix{
   {a_1 } & {a_2 } & {a_3 } &  \ldots  & {a_X }  \cr 
   {b_1 } & 0 & 0 & 0 & 0  \cr 
   0 & {b_2 } & 0 & 0 & 0  \cr 
   0 & 0 &  \ddots  & 0 & 0  \cr 
   0 & 0 & 0 & {b_{X - 1} } & 0  \cr 
 } } \right), $$

to build which we need to know $a_x$, {\em ability to reproduce}, an average number of offsprings within a time step from an individual of age class $x$, and $b_x$, {\em probability of surviving}, a fraction of individuals that survives from age class $x$ to age class $x+1$.
 
 \stoptext
%%%%%%%%%%%%

and a matrix has big spacing BEFORE and normal spacing AFTER. 

Interesting, but without two first setup lines, matrix glues to the subsequent text. 

Best,
Vaytcheslav