Bill, I changed my mind about retractIfCan/RetractableTo, so
Bill, Ralf and me should on the same page.
About 'coerce : R -> Maybe R', this auto-coercion only works at
interpreter, right? So when writing Spad, one should explicitly
use 'wrap' (or 'coerce', but 'wrap' is more clear).
About 'retract : Maybe R -> R', it should do error checking, as other
'retract' do. And when writing Spad, one can use 'unwrap(x)' or
'retract(x)', but not 'x::R' like we used to when using Union("failed",...),
or should we? By adding 'convert : Maybe R -> R' (with error checking)?
A lot of current code use :: to retract from Union("failed", R) to R,
I experienced that in the previous commit.
I'm inclined to have this '::' conversion, because '::' is still explicit.
Frankly, the compiler trick that let you omit the '::' conversion is not
used many times, and that trick is not always available.
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