Hi I'm looking at cocoon and other property systems. I would like to get a nice library once for all. I want to also revisit Artefact stylesheets. I'm checking the semantics regarding the lookup of a property in a parent when it is not found in the receiver. I have the impression that when there is a parent access and modification should modify the same object: i.e., the parent if the key is found in the parent or the child if the property is found in the child. in cocoon we have a mix of the two semantics and I would like to get your feel about it. propertyAt: aKey "Answer the value of the property ==aKey==, raises an error if the property doesn't exist." ^ self propertyAt: aKey ifAbsent: [ self hasParent ifTrue: [ self parent propertyAt: aKey ] ifFalse: [self errorPropertyNotFound: aKey ]] propertyAt: aKey default: aBlock "Answer the value of ==aKey==. If the key have no entry I look on the parent or execute a block if i'm an orphan." ^ self propertyAt: aKey ifAbsent: [ self hasParent ifTrue: [ self parent propertyAt: aKey default: aBlock ] ifFalse: aBlock ] but propertyAt: aKey ifAbsent: aBlock "Answer the value of the property ==aKey==, or the result of ==aBlock== if the property doesn't exist." ^ self properties at: aKey ifAbsent: aBlock propertyAt: aKey ifAbsentPut: aBlock "Answer the value of the property ==aKey==, or if the property doesn't exist adds and answers the result of evaluating ==aBlock==." ^ self properties at: aKey ifAbsentPut: aBlock In PetitParser there is no lookup. But we can get back to this situation by just not setting a parent. My gut feeling is telling me that overriding on write is not really good because a client of the parent will not see the changes coming from a child. I thaought that stylesheet was supporting lookup but it does not testSubstyleAttributesOverride styleSheet at: #textColor put: #red. styleSheet > #sub > #subsub at: #textColor put: #blue. self assert:( styleSheet get: #textColor ) equals: #red. self assert:( styleSheet > #sub get: #textColor ) equals: #red. self assert:( styleSheet > #sub > #subsub get: #textColor ) equals: #blue. Does not work What you think? Stef