Reserved keyword rule

[eileenjang]

Function
: first
;

first
: '(' (STRING) (',' firstMode)? ')'
;

firstMode
: A
| B
;

A
: '"' F I R S T '"'
;

B
: '"'F I R S T D A Y'"'
;

STRING
: '"' .*? '"'
;

How to set "FIRST" as A only in the context of first but set "FIRST" as STRING out of that context?
e.g type only "FIRST" -> STRING
type ("example", "FIRST") -> A