我想设立一个结构方程模型（sem）的循环，用的lavvan包，但是他这样报错怎么办？报错：Error in lavParseModelString(model) :
  lavaan ERROR: left hand side (lhs) of this formula:
    mydata[,i+19] ~ mydata[,i]+mydata[,i+1]
    contains either a reserved word (in R) or an illegal character: “mydata[,i”“19]”
    See ?reserved for a list of reserved words in R
    Please use a variable name that is not a reserved word in R
    and use only characters, digits, or the dot symbol.