Question: inconsistency in expanding (a+b)^n

Hi Maple friends.

expand( (a+b)^2 );

a^2+2*a*b+b^2

expand( (a+b)^3 );

a^3+3*a^2*b+3*a*b^2+b^3

expand( (a+4)^4 )

a^4+16*a^3+96*a^2+256*a+256 (???)

Pascal's triangle shows that 'expand( (a+4)^4 )' should have resulted in a^4+4*a^3*b+6*a^2*b^2+4*a*b^3+b^4

Where are the b variables in Maple's solution?

Thanks in advance.

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