EXPRESS data modeling language is an actively used meta language. EXPRESS is a standard data modeling language for product data. EXPRESS is formalized in the ISO Standard for the Exchange of Product model STEP (ISO 10303), and standardized as ISO 10303-11.. Read more on Wikipedia...

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Example code from the web:

SCHEMA Family;

ENTITY Person
   ABSTRACT SUPERTYPE OF (ONEOF (Male, Female));
     name: STRING;
     mother: OPTIONAL Female;
     father: OPTIONAL Male;
END_ENTITY;

ENTITY Female
   SUBTYPE OF (Person);
END_ENTITY;

ENTITY Male
   SUBTYPE of (Person);
END_ENTITY;

END_SCHEMA;

Last updated February 18th, 2020

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