`col : %col`
============
Tuples
------
The `:` runes construct [tuples]().
There is no natural `:` rune. Instead, all of them derive from the
autocons property of `++twig`, as show below.
++ twig $& [p=twig q=twig]
Namely, a cell of two twigs is a twig producing a cell of the results of
the two original sub-twigs.
The `:` runes produce one of three categories of tuples
#### Tuples of determinate size:
[`:-`]() Tuples of two elements, `p` and `q`. [`:_`]() Tuples of two
reversed elements, `q` and `p`. Used to ensure vertical code flow.\
[`:+`]() Tuples of three elements `p`, `q`, and `r`. [`:^`]() Tuples of
four elements `p`, `q`, `r`, and `s`.
#### Tuples of indeterminate size:
[`:*`]() Tuples of n elements. [`:~`]() Null-terminated tuples of n
elements.
#### Tuples used for interpolation:
[`:/`]() Tuple designed to be interpolated into an XML structure.