merge
[s1] ----a-----b--c---->
[s2] o----------------->
s3 = s1.merge(s2)
[s3] o---a-----b--c---->
startsWith, merge
[s1] ----u--------u--->
s1.startsWith(N)
[s2] N---u--------u--->
[s3] -------r---r----->
s2.merge(s3.map(e => N))
[s3] N---u--N---N-u--->