1    %let L1=L2;
2    %let L2=L3;
3    %let L3=L4;
4    %let L4=L5;
5
6    %put &L1;
结果：L2

7    %put &&L1;
结果：L2
过程：&&L1 -> &L1 -> L2

8    %put &&&L1;
结果：L3
过程：&&&L1 -> &L2 -> L3

9    %put &&&&L1;
结果：L2
过程：&&&&L1 -> &&L1 -> &L1 -> L2


10   %put &&&&&L1;
结果：L3
过程：&&&&&L1 -> &&L2 -> &L2 -> L3


11   %put &&&&&&L1;
结果：L3
过程：&&&&&&L1 -> &&&L1 -> &L2 -> L3


12   %put &&&&&&&L1;
结果：L4
过程：&&&&&&&L1 -> &&&L2 -> &L3 -> L4


13   %put &&&&&&&&L1;
结果：L2
过程：&&&&&&&&L1 -> &&&&L1 -> &&L1 -> &L1 -> L2


14   %put &&&&&&&&&L1;
结果：L3
过程：&&&&&&&&&L1 -> &&&&L2 -> &&L2 -> &L2 -> L3


15   %put &&&&&&&&&&L1;
结果：L3
过程：&&&&&&&&&&L1 -> &&&&&L1 -> &&L2 -> &L2 -> L3

========= SAS 代码 =========
options symbolgen;
%let L1=L2;
%let L2=L3;
%let L3=L4;
%let L4=L5;
%put &L1;
%put &&L1;
%put &&&L1;
%put &&&&L1;
%put &&&&&L1;
%put &&&&&&L1;
%put &&&&&&&L1;
%put &&&&&&&&L1;
%put &&&&&&&&&L1;
%put &&&&&&&&&&L1;


========= SAS 日志 =========
5780   options symbolgen;
5781
5782   %let L1=L2;
5783   %let L2=L3;
5784   %let L3=L4;
5785   %let L4=L5;
5786
5787   %put &L1;
SYMBOLGEN:  Macro variable L1 resolves to L2
L2
5788   %put &&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
L2
5789   %put &&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
SYMBOLGEN:  Macro variable L2 resolves to L3
L3
5790   %put &&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
L2
5791   %put &&&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L2 resolves to L3
L3
5792   %put &&&&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
SYMBOLGEN:  Macro variable L2 resolves to L3
L3
5793   %put &&&&&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L2 resolves to L3
SYMBOLGEN:  Macro variable L3 resolves to L4
L4
5794   %put &&&&&&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
L2
5795   %put &&&&&&&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L2 resolves to L3
L3
5796   %put &&&&&&&&&&L1;
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L1 resolves to L2
SYMBOLGEN:  && resolves to &.
SYMBOLGEN:  Macro variable L2 resolves to L3
L3