<Ul> <Li> All material terms of the contract must be in writing . </Li> <Li> Terms implied by law (naturalia), as well as tacit terms, need not be in writing . </Li> <Li> The terms need not all be in one document . </Li> <Li> Any variation of a material term of the contract has to be in writing to be effective . An extension of time, a cancellation of contract and the revival of a cancelled contract do not amount to variations . </Li> <Li> The defence of estoppel may not be raised where a party has been misled to believe that there has been an oral variation of the contract . </Li> <Li> If formalities are not complied with, the contract is void . The return of a performance in a void contract may be claimed with an enrichment action . An alienation of land is valid from the beginning, if both parties have performed fully . </Li> </Ul> <Li> All material terms of the contract must be in writing . </Li> <Li> Terms implied by law (naturalia), as well as tacit terms, need not be in writing . </Li> <Li> The terms need not all be in one document . </Li>

Name 2 naturalia of a contract of purchase and sale
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