Assertion: melanin | /r/HasProperty | defined in several ways
Facets:
value: during the last 50 years
type: TEMPORAL
Source sentences:
text: They have been defined in several ways during the last 50 years [8–10] but most of the proposed definitions present some small pitfalls as they are partial or uncompleted due to the difficulty of defining something with such a wide diversity in composition, color, size, occurrence, and functions.