Nested foreach

I am trying to understand the working behaviour of foreach . i have tried this below code from appian doc.

a!forEach(
items: {"January", "February", "March"},
expression: a!localVariables(
local!month: fv!item,
a!forEach(
items: {1, 15},
expression: local!month & " " & fv!item
)
)
)

the code throws the below error.Kindly help.