When green is the map colour what is the future stock?
CREATE TABLE table_21957 (
    "Name" text,
    "Map colour" text,
    "First operated" real,
    "Type" text,
    "Length" text,
    "No. Sta" real,
    "Current Stock" text,
    "Future Stock" text,
    "Trips per annum (\u00d71000)" real,
    "Avg. trips per mile (\u00d71000)" real
)
SELECT "Future Stock" FROM table_21957 WHERE "Map colour" = 'Green'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: When green is the map colour what is the future stock? ### Input: CREATE TABLE table_21957 (
    "Name" text,
    "Map colour" text,
    "First operated" real,
    "Type" text,
    "Length" text,
    "No. Sta" real,
    "Current Stock" text,
    "Future Stock" text,
    "Trips per annum (\u00d71000)" real,
    "Avg. trips per mile (\u00d71000)" real
) ### Response: SELECT "Future Stock" FROM table_21957 WHERE "Map colour" = 'Green'