how to join list of maps in groovy -

if have 2 list of maps in groovy...

def x = [ [a:1, b:2], [a:1, b:3], [a:2, b:4] ] ​def y = ​[ [f:10, b:2, g:7], [f:100, b:3, g:8], [f:20, b:4, g:9] ] 

how can join them based on particular attribute. in above example values b.

desired result is...

[a:1, b:2, f:10, g:7] [a:1, b:3, f:100, g:8] [a:2, b:4, f:20, g:9] 

i tried it's not i'm after.

def z = (x + y).groupby { it.b } z.each{ k, v -> println "${k}:${v}" } 

thanks

can't do:

[x,y].transpose().collect { a, b -> + b }