We present mstlo (mistletoe), a Rust library for high-performance online monitoring of signal temporal logic (STL), with Python bindings. The library provides: (i) a unified interface for multiple STL semantics, including Robust Satisfaction Intervals (RoSI) and Boolean evaluation with early verdicts; (ii) an incremental monitoring algorithm based on bottom-up dynamic programming with per-operator caching and streaming extremum computation for temporal operators; and (iii) an embedded STL domain-specific language for both Rust and Python implementations, with procedural macros in Rust for static syntax checking. Benchmarks show scalability and performance improvements over state-of-the-art tools, especially for formulas with large temporal depth and deep nesting.

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