Fix parser for "as" rules

[krtek4]

Fix the parser for the following patterns :

• '[a, b][c, d]' : shouldn't be valid
• '(x:xs)@(x:xs)' : catched by the cons rule instead of generating an error
• '[a, b]@[c, d]' : catched by the array rule instead of generating an error

The last two could eventually make sense, but they need to be fixed anyway.