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Ambiguous conditions

correct block read was not possible:
Typically results in a throw of some OS-derived value (between -512 and -2048). If the blocks file was just not long enough, blanks are supplied for the missing portion.
I/O exception in block transfer:
Typically results in a throw of some OS-derived value (between -512 and -2048).
invalid block number:
-35 throw (Invalid block number)
a program directly alters the contents of BLK:
The input stream is switched to that other block, at the same position. If the storing to BLK happens when interpreting non-block input, the system will get quite confused when the block ends.
no current block buffer for UPDATE:
UPDATE has no effect.