s := "abcd" ----> "abcd"
{s[1:2] := "xx";s} ----> "xxbcd"
{s[-1:0] := "";s} ----> "xxbc"
{s[1] := "abc";s} ----> "abcxbc"
{s[1+:2] := "y";s} ----> "ycxbc"
{s[2] :=: s[3];s} ----> "yxcbc"
s[6] := "t" ----> none
s[0-:6] := "u" ----> none
{s[1:0] :=: s[0:1];s} ----> "yxcbc"
Run-time error 103 at line 18 in expr12.icn
string expected
offending value: &null