What if we want to put an action in the middle of the rhs of a rule in a bottom-up parser? In that case we use a dummy nonterminal, called a marker. For example,

X ::= a { action } b

is equivalent to

X ::= M b

M ::= a { action }

This is done automatically by the CUP parser generator (ie, we can actually put actions in the middle of a rhs of a rule and CUP will use the above trick to put it at the end of a rule). There is a danger though that the resulting rules may introduce new shift/reduce or reduce/reduce conflicts.