Ensure user-defined preconditions run after speced.def's

[vemv]

Problem statement

Given a speced defn with user-provided :preconditions:

• both sources of preconditions will be correctly merged; but
• the user ones take precedence.

This means that more opaque errors can pop up: type (speced's) errors are more straightfoward than a business-specific one (user's) that failed for 'type' reasons.

Task

Merge preconditions in a specific order.