Currently all symbols defined by the user are written to the settings.xml upon
saving the formula or text document. Thus if a user has a Writer document with
say 300 formula and has a set of 100 user-defined defined symbols those 100
symbols will be added to each formula even if that does not make use of any symbol. 
Thus to reduce document the size the settings.xml should only list those symbols
that are used by the respective formula.

Another effect of the current behavior is that even on different
computers/office-installations than the one where the document got created all
user-defined symbols from that original computer will be visible in the symbol
catalogs of other computers when the formulas get edited. 
By only listing the actually used ones in the settings.xml the symbol catalog on
different computer also will no longer list all the user-symbols that were
previously listed when a formula is edited. 
This may look to some users like a loss of functionality, thus we should also
introduce an UI option that allows to specify if all symbols or only those in
use should be saved to file/formula.