lazy rule SubtractAtom2Term {
    from
    	source: transformate!SubtractAtom
    to
    	otxTerm: otx!Subtract(
                -- This works as desired, i.e. the quantity goes to output.otx:
                numeral <- source.numeral->createQuantity(),
                -- This does not work, i.e. the quantities go to output.quantity:
                subtrahend <- source.subtrahends->collect(s | s->createQuantity())
                -- Error message: Containment references cannot span across models.
    	)
}

helper context transformate!QuantityVariable def: createQuantity(): quantity!Quantity = thisModule->QuantityVariable2Quantity(self);

lazy rule QuantityVariable2Quantity {
    from    
        source: transformate!QuantityVariable
    to
        output: quantity!Quantity(
            -- ...
        )
}

-- org.iso.otx.quantities is a sub-model of org.iso.otx
create OUT: otx, quant: quantities from IN: transformate;

-- ...

lazy rule QuantityVariable2Quantity {
    from    
        source: transformate!QuantityVariable
    to
        output: quantity!Quantity in OUT(
            -- ...
        )
}