li = list()

var = "continent"
val = "Europe"
li = append(li, quo(!!sym(var) %in% !!val))

var = "year"
val = 1980
li = append(li, quo(!!sym(var) <= !!val))

li

> li
[[1]]
<quosure>
  expr: ^continent %in% "Europe"
env:  global

[[2]]
<quosure>
  expr: ^year <= 1980
env:  global

li = list()

var = "continent"
val = "Europe"
li = modifyList(li, list(quo(!!sym(var) %in% !!val)))

var = "year"
val = 1980
li = modifyList(li, list(quo(!!sym(var) <= !!val)))

li
> li
list()