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qr.solve <- function(a, b, tol = 1e-7)
{
    if( !is.qr(a) )
    a <- qr(a, tol = tol)
    nc <- ncol(a$qr)
    if( a$rank != nc )
    stop("singular matrix `a' in solve")
    if( missing(b) ) {
    if( nc != nrow(a$qr) )
        stop("only square matrices can be inverted")
    b <- diag(1,nc)
    }
    ## pre 0.63.3: b <- as.matrix(b)
    return(qr.coef(a,b))
}

solve <- function(a, b, ...) UseMethod("solve")
solve.default <- .Alias(qr.solve)
solve.qr <- .Alias(qr.solve)