A study in Sweden indicated that kindergarten children who could play in a natural environment had less illness and greater physical ability than children used only to a normal playground. 请问老师这里的used only to语法上应该怎么理解呢？"习惯"的表达不是"be used to sth"吗，这个表达法可以做后置定语吗？答案解析说省略了"who are"，请问为什么可以省略呢？省略之后有点不能理解它是"习惯"的意思。