Simplify Primitives

[DavePearce]

The fundamental primitives currently in schema/constraint and schema/assignment include various things which are not actually primitives. What we want are just the actual fundamental primitives:

• Data Columns

• Computed Columns. These apply an arbitrary computation to a given set of source columns, producing a given set of target columns.

• Lookup Constraints

• Permutation Constraints

• Vanishing Constraints

Everything else is really something at a higher level (e.g. a sorting constraint). For example, the lexicographic sorting constraint (over say columns X, Y, Z) is really a vanishing constraint which says this:

X[i-1] < X[i]
|| (X[i-1] == X[i] && Y[i-1] < Y[i]) 
|| (X[i-1] == X[i] && Y[i-1] == Y[i] && Z[i-1] < Z[i])  
|| (X[i-1] == X[i] && Y[i-1] == Y[i] && Z[i-1] == Z[i])

Implementing this naively however, does not result in the same optimised column representation.