Pre-condition: object must be visible. This condition doesn't attempt any implied command to make the object visible, but merely enforces visibility before allowing the command.

This condition is useful for commands that rely on visibly inspecting the object, such as "examine" or "look in". It is possible for an object to be in scope without being visible, since an object can be in scope by way of a non-visual sense.

We enforce visibility with a verification test, not a precondition check.

objVisible :   PreCondition