The support for default types has been improved up.

Now updatable is able to use default types avoiding workarounds (like
adding dummy alias at object creation). The magic is done by the
'substituteSymbols' method, which empowers us to replace from normal
types to default ones.
Now, "overridden" types are supported.

I noticed that default types were failing in 'Context'. It seems that
overridding types was breaking the scheme. I have added some magic
lines to avoid that case.