s2 := "humongous"
  s3 := "little"
  s1 :=  "Mary had a humongous lamb."
  s1 ?:= tab(find(s2)) || (=s2,s3) || tab(0)          # replaces the first instance of s2 with s3
  while s1 ?:= tab(find(s2)) || (=s2,s3) || tab(0)    # replaces all instances of s2 with s3, equivalent to replace