An event source whose current value or state is readable
    Note that the firing behavior of an IReadableSource is undefined.  See
    IValue and IConditional for two possible kinds of firing behavior. 
        
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                  Methods 
                
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__call__ 
derive 
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                __call__ 
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__call__ ()
 
Return the current value, condition, or event 
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                derive 
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derive ( func )
 
Return a derived IValue, computed using func(source()) 
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