seq vs array vs open array

[cblake]

It also works if you add this to the end of @shirleyquirk's program:
    
    
    assert @x & @y & @z == [1,2,3,4,5,6]
    
    
    Run

As well as if you put `@` before `[1,2,3,4,5,6]` either above **_or_** for 
`foo` from `arrayConcat`.

An "inverse `@`" (`toArray`, say) would probably let you get by with one set of 
CT manipulators in terms of type/signature matching. Run-time is very 
different, of course. `const` s barely have addresses, heap|elsewise.