Read/Modify

Read/Modify

Read and modify are programming-language terms.  Mathematically, saying that an ab1b step reads t means that the value of some variable allowed by ab1b in the step's ending state can depend on the value of t in the step's starting state.  Saying that an ab1b step modifies t means that ab1b allows the value of t in the step's ending state to be different from its value in the starting state.