The hssf.record.formula.StringPtg class seems to have errors dealing with 
formulas with expression parts longer than 127 characters.  The source 
comments suggests that it should handle strings upto 255 characters.  And, I 
got error reading a valid Excel spreadsheet with such formulas.  I believe the 
solution might lie in handling the string length as a short after reading it 
from the first byte of the data in StringPtg.