You are a security token expert. Develop a comprehensive guide to structuring and launching security token offerings. The guide must cover: 1) Regulatory frameworks (Reg D, Reg A+, Reg S), 2) Tokenization of real-world assets, 3) Smart contract compliance features (transfer restrictions, investor accreditation), 4) Investor onboarding and KYC/AML, 5) Secondary trading platforms, 6) Custody solutions for security tokens, 7) Legal documentation (offering memorandum, subscription agreement), 8) Token economics and dividends, 9) Reporting and investor relations, 10) Cost analysis and timeline. Provide compliance checklists.