```
data(lsq)
class(lsq) # -> [1] "matrix.csc.hb"
model.matrix(lsq)->design.o
class(design.o) # -> "matrix.csr"
dim(design.o) # -> [1] 1850 712
y <- model.response(lsq) # extract the rhs
length(y) # [1] 1850
X <- as.matrix(design.o)
c(object.size(X) / object.size(design.o)) ## X is 92.7 times larger
t(design.o) %*% design.o -> XpX
t(design.o) %*% y -> Xpy
chol(XpX) -> chol.o
determinant(chol.o)
b1 <- backsolve(chol.o,Xpy) # least squares solutions in two steps
b2 <- solve(XpX,Xpy) # least squares estimates in one step
b3 <- backsolve(chol.o, forwardsolve(chol.o, Xpy),
twice = FALSE) # in three steps
## checking that these three are indeed equal :
stopifnot(all.equal(b1, b2), all.equal(b2, b3))
```

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