[metapost] Unable to figure out "if"

[Troy Henderson]

for i=-60 step 10 until 61:
   if (i > -31) and (i < 31):
      anta:= 70;
   else:
      anta:= 60;
   fi;
   draw (-anta,i)--(anta,i);
   draw (i,-anta)--(i,anta);
endfor;

--
Troy Henderson
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