[metapost] Can not use boxes.mp -- capacity exceeded

[Steven Woody]

it seemed my metapost can not work with boxes. if i use "input boxes",
metapost will definitely compain:

(boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp
(boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp (boxes.mp
(boxes.mp (boxes.mp (boxes.mp (boxes.mp
! MetaPost capacity exceeded, sorry [text input levels=20].
l.7 input boxes


what's the hell going on? thanks.

-- 
woody

then sun rose thinly from the sea and the old man could see the other
boats, low on the water and well in toward the shore, spread out
across the current.