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I am trying to identify the appropriate fragment of Datalog containing rules of the type:

A(x,y):-B(x,z),c(z,y)

where B(x,z) is an IDB relation and c(z,y) is EDB relation. The special property of this fragment is the each free variable (i.e. a variable not in the head, z in this case) must appear at least twice. Put differently, the following rule is illeagal:

A(x,y):-B(x,y),c(x,z)

What is the name of this Datalog fragment and is there any complexity upper and/or lower bounds?

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