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circle <- function(xorig, yorig, radius, add, ...){
  
  x <- seq(-radius, radius, length.out = 1000)
  
  y <- sapply(x, function(z) sqrt(radius^2 - z^2))
  
  if(add == TRUE){
    
    lines(xorig + c(x, rev(x)), c(yorig + y, yorig + rev(-y)),
          type = "l", ...)
    
  } else {
    
    plot(xorig + c(x, rev(x)), c(yorig + y, yorig + rev(-y)),
         type = "l", ...)
    
  }
}

mo.y <- rep(0.15,100)
mo.x <- seq(0.2,0.8,length=100)

circle(0.3, 0.75, 0.20, add = FALSE,
       main = "A Mo",
       xlab = "",
       ylab = "", ylim=c(-0.5,1), xlim=c(0,1), col="red",lwd=4)
circle(0.5,0.25,0.55,add=TRUE,col="black",lwd=4)
circle(0.72, 0.75, 0.20, add = TRUE, col="red",lwd=4)
circle(0.5, 0.5, 0.18, add = TRUE, col="orange",lwd=4)
circle(0.3, 0.75, 0.10,add=TRUE,col="black",lwd=4)
circle(0.72, 0.75, 0.10,add=TRUE,col="black",lwd=4)
lines(mo.y~mo.x,lwd=4)