Contains tests whether the sequence of characters in the right operand is a substring of the left operand. If the left operand contains the character string represented by the right operand, the result is TRUE (1). If the left operand does not contain the character string represented by the right operand, the result is FALSE (0). If the right operand is the null string, the result is always TRUE.
For example:
SET L="Steam Locomotive"
SET S="Steam"
WRITE L[S /// 1
For details, see the Contains ([) reference page. Also see the Does Not Contain ('[) reference page.