go =>
S1 = "string",
println(s1=S1),
S2 = S1,
S2[1] := 'x', % also changes S1
println(s2=S2),
nl,
S3 = "string",
S4 = copy_term(S3),
S4[1] := 'x', % no change of S3
println(s3=S3),
println(s4=S4),
nl.