Must read! Really good explanation of MAST by
This is the best explanation of MAST I've read anywhere: (disclaimer: I also write at bitcointechtalk)
Clever idea: “What is a Bitcoin Merklized Abstract Syntax Tree (MAST)?” — 
“What is a Bitcoin Merklized Abstract Syntax Tree (MAST)?” — 
"What is a Bitcoin Merklized Abstract Syntax Tree (MAST)?" by