typed enclosures

4.29: mis.adda/syntax/typed enclosures:
. adda currently reserves the square brackets
as a way to allow more freedom in identifier spellings:
. the space and all printable characters are allowed
as long as every use of square brackets is paired,
because then the ending bracket is well-defined;
eg, [*thi{ i$ ()ne [eg@l iden]ifier] .
. one way to allow multiple uses for square brackets
is to reserve typed brackets: eg,
[this is a single symbol] --. no typing syntax .
[.M(2,3): ...] --. this is typed as being a 2x3 matrix .
{.R: [a,b)} -- a <= x < b for x in reals .
. this seems too arbitrary, with no compelling use cases ...
. however, look at the angle brackets:
while the type is .< ... >
the other use is as a value:
<. ... > or <.type: ...>
and of course the 3rd use is as the less-than,
without any dots nearby .