types. The notion of "unknown type" is different in function of the target
object:
  - a DBType and a FunctionDef have an unknown type if their function
    isUnknown() returns true. In such case, the other functions such as
	isNumeric/String/Geometry() will return false.
  - an ADQLOperand (e.g. ADQLColumn) does NOT have a isUnknown() function.
    But if the type of the operand is unknown, its functions isNumeric(),
	isString() and isGeometry() must ALL return true. Otherwise, just one of
	these functions can return true.