IO-shareable is a general recursive defintion including:

• matrix type
• arrays
• structures (with arbitrary nesting)

At some point we simplified assumptions.

• Builtin inputs are only scalars or numeric vectors.
• User data inputs are only, numeric scalars or numeric vectors
  Pipeline IO parameters and returns are either: any of the above, or a structure of the above (without nested structures).

We can simplify IO-shareable:

• remove matrix
• remove array
• allow only singly-leveled structures.

Consider deleting the definition if it's not pulling its editorial weight.