Read Operation
Weikum本 p121 Bibliographic Notesによると: the subtle difference between these viewpoints already comes up in the definition of reading a value from a write step, which has to be written as follows when aborted transactions are taken into account:
A step $ r_i(x) \in s of a transaction $ t_i \in trans(s) reads the value written by the last $ w_j(x) \in s, $ j \neq i,that occurs before $ r_i(x),such that $ a_j \notin s.
occurs beforeというのはつまり$ w_j(x) <_s r_i(x)ということ．