input <- c(
    "a1=a2", "b1=b2", "b3=b2", "c1=c2", "e1=e2", "a3=a4", "c3=c4", "e1=e3",
    "a2=a4", "c3=c1", "b3=a4", "c2=d1", "a4=a5", "d2=c1", "b4=b3", "d3=c3"
)
 
eq <- strsplit(input, "=")
id <- unique(unlist(eq))
n.id <- length(id)
 
A <- matrix(0, n.id, n.id)
rownames(A) <- colnames(A) <- id
E <- t(matrix(unlist(eq), 2))
A[E] <- A[E[, 2:1]] <- 1
 
group <- rep(0, n.id)
n.group <- 0
for (i in 1:n.id) {
    if (group[i]) next
    group[i] <- n.group <- n.group + 1
    while (length(i)) {
        j <- which(colSums(A[i, , drop = FALSE]) != 0)
        i <- j[!group[j]]
        group[i] <- n.group
    }
}
 
for (i in 1:n.group) cat("[", toString(id[group == i]), "]\n", sep = "")

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