% no working directory
adding a
adding b
% hg parents
changeset:   2:6cfac479f009
tag:         tip
user:        test
date:        Thu Jan 01 00:00:02 1970 +0000
summary:     b

% hg parents a
changeset:   0:b6a1406d8886
user:        test
date:        Thu Jan 01 00:00:00 1970 +0000
summary:     ab

% hg parents -r 2
changeset:   1:d786049f033a
user:        test
date:        Thu Jan 01 00:00:01 1970 +0000
summary:     a

% hg parents -r 2 a
changeset:   0:b6a1406d8886
user:        test
date:        Thu Jan 01 00:00:00 1970 +0000
summary:     ab