If I have two different \startMPcode ... \stopMPcode sections, is
there a way (perhaps it does this by default) to have the variables
stored in the first block available in the second block?  In pure
MetaPost, I can do

numeric a:=4;
beginfig(1);
   label(decimal(a),origin);
endfig;
beginfig(2);
   label(decimal(a),origin);
endfig;
end

and both figures have the value of 'a' available to them because 'a'
was defined before each figure.  What is the ConTeXt way of doing
this?