Lazy-CSeq-SP: boosting sequentialization-based verification of multi-threaded C programs via symbolic pruning of redundant schedules. Sequentialization has been shown to be an effective symbolic verification technique for concurrent C programs using POSIX threads. Lazy-CSeq, a tool that applies a lazy sequentialization scheme, has won the Concurrency Division of the last two editions of the Competition on Software Verification. The tool encodes all thread schedules up to a given bound into a single non-deterministic sequential C program and then invokes a C model checker. This paper presents a novel optimized implementation of lazy sequentialization, which integrates symbolic pruning of redundant schedules into the encoding. Experimental evaluation shows that our tool outperforms Lazy-CSeq significantly on many benchmarks.