In a Japanese children's book I read:
のりまきみたいに くるまれば ふかふか ふとんに なるんだもん。
For the sake of analysis, I looked up the kanjis (correct me if they're wrong):
From the context, it should mean something along the lines, "I can wrap it up like a Nori roll and it becomes a soft mattress."
However, two things are unclear to me: First, is くるまれば really a form of 包まる? If so, what verb form is it? jisho.org does not have it in its list of inflections. What does this form mean, when do I use it?
Second, what does もん mean? Is it a shortened form of 物? How does this word alter the sentence?