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.