ICC经常会出现flat，和hierarchical

例如all_fanout -flat
Specifies to function in the flat mode ofoperation.Thetwo
majormodesinwhichall_fanoutfunctionsare hierarchical
(default) and flat.When inhierarchicalmode,onlyobjects
fromthesamehierarchylevelasthecurrentsourceare
returned.Thus, pins within a levelofhierarchylowerthan
thatof the source are used for traversal but are not reported.
看的云里雾里的，新人求解释

指的是 -flat 可 trace print 穿越所有 Hirearchy level Leaf cell pin, default 是在同一個 level 的 leaf cell pin 才被 print 出來.