irb(main):001:0> a=[0,1,2]
=> [0, 1, 2]
irb(main):002:0> [a]
=> [[0, 1, 2]]
irb(main):003:0> [*a]
=> [0, 1, 2]
irb(main):004:0> [3,4,5,*a]
=> [3, 4, 5, 0, 1, 2]
irb(main):005:0> [*a,3,4,5]
SyntaxError: compile error
(irb):5: syntax error, unexpected ',', expecting ']'
[*a,3,4,5]
^
from (irb):5

4行目は良いのに5行目はダメっていう挙動が理解できないんだが・・・