Clean up and test PropertyHelpers

[GoogleCodeExporter]

Clean up and test PropertyHelpers. Use exception vernacular. Think about the 
methods (are there 
too many?) Remove remaining exceptions. These are programming errors, not 
exceptions.

Original issue reported on code.google.com by jandockx on 28 Jul 2008 at 9:59

[GoogleCodeExporter]

Original comment by jandockx on 28 Jul 2008 at 9:59

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