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Design references:

• SQL NULL on Wikipedia
• Working with missing data in the Pandas manual
• Missing Data Functionality in NumPy enhancement proposal (2011)
• Pandas 2 Missing data consistency design document
• Missing Values in the SAS manual
• Missing Values in R, Stata, SAS and SPSS on UCLA's IDRE website
• Types of missing values (NullFlavor) in the Health Level 7 v3 standard.