What are the 4 valid contracts?
Could you please elaborate on the concept of "What are the 4 valid contracts?"? I'm interested in understanding what exactly these contracts represent and how they operate within the realm of cryptocurrency and finance. Could you provide examples or scenarios where these contracts might be utilized? Additionally, how do these contracts ensure validity and what are the key features that distinguish them from other types of agreements? I'm eager to gain a deeper understanding of this topic.