Pattern Matching BlankSequence

Question

Consider the definition:
Clear[fun]
fun[a_ + b_] := fun[a] + fun[b];
fun[a+b+c]
(*fun[a] + fun[b] + fun[c]*)

This works as expected. However, if we use a BlankSequence on "b":
Clear[fun]
fun[a_ + b__] := fun[a] + fun[b];
fun[a + b + c]
(*fun[a] + fun[b, c]*)

it seems that  BlankSequence eliminates the Plus in "b+c" and replaces it with Sequence.
Has anybody a good explanation for this behaviour?

wuyudi · Accepted Answer

a+b+c == Plus[a,b,c]
So fun[a_ + b__]  == fun[Plus[a_,  b__]]
when input a + b + c,which equals Plus[a,b,c] , mma match b with b,c
a + b + c /. a_ + b__ :> {a, {b}}

{a, {b, c}}

Pattern Matching BlankSequence

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