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PURE and IMPURE functions.
Functions in VHDL'93 can be labeled PURE
or IMPURE. Functions labeled PURE may not access data outside the function or
access files. WARNING: VHDL'87 file declarations may cause errors because
functions are
assumed to be PURE in VHDL'93. Convert these files to VHDL'93 by adding the
keyword IMPURE before the keyword FUNCTION, or compile them with the -87
option.