Using xargs instead of GNU Parallel

Question

GNU Parallel can generate all combinations of input, copy shell functions and run them on 3 remote servers without mixing the output, retrying the command if it fails:

#!/bin/bash

env_parallel --session
process_output() {
  perl -ne '$/=undef; $|=1; @a=split//;
      while(@a){
        print ".",shift @a;
        select($a,$a,$a,0.100);
      }';
}
env_parallel --retries 4 -Sserver{1..3},: \
  'echo X={1} Y={2} Z={3} | process_output' \
  ::: "a'  \"b"{1..3} ::: "c\"  'd"{3..4} ::: "e\$  f'"{7..8}

Output:

.X.=.a.'. . .".b.1. .Y.=.c.". . .'.d.3. .Z.=.e.$. . .f.'.7.
.X.=.a.'. . .".b.1. .Y.=.c.". . .'.d.3. .Z.=.e.$. . .f.'.8.
.X.=.a.'. . .".b.1. .Y.=.c.". . .'.d.4. .Z.=.e.$. . .f.'.7.
.X.=.a.'. . .".b.1. .Y.=.c.". . .'.d.4. .Z.=.e.$. . .f.'.8.
.X.=.a.'. . .".b.2. .Y.=.c.". . .'.d.3. .Z.=.e.$. . .f.'.7.
.X.=.a.'. . .".b.2. .Y.=.c.". . .'.d.3. .Z.=.e.$. . .f.'.8.
.X.=.a.'. . .".b.2. .Y.=.c.". . .'.d.4. .Z.=.e.$. . .f.'.7.
.X.=.a.'. . .".b.2. .Y.=.c.". . .'.d.4. .Z.=.e.$. . .f.'.8.
.X.=.a.'. . .".b.3. .Y.=.c.". . .'.d.3. .Z.=.e.$. . .f.'.7.
.X.=.a.'. . .".b.3. .Y.=.c.". . .'.d.3. .Z.=.e.$. . .f.'.8.
.X.=.a.'. . .".b.3. .Y.=.c.". . .'.d.4. .Z.=.e.$. . .f.'.7.
.X.=.a.'. . .".b.3. .Y.=.c.". . .'.d.4. .Z.=.e.$. . .f.'.8.

xargs -P$(nproc) can run one job per processor.

Given

• the function process_output,
• the servers to run on (server1, server2, server3),
• the argument groups (a' "b1, a' "b2, a' "b3;c" 'd1, c" 'd2, c" 'd3;e$ f'7, e$ f'8)

how can you emulate GNU Parallel using xargs to get the same done using process_output and the servers?