CREATE TABLE "year_by_year" (
  "year" real,
  "division" real,
  "league" text,
  "reg_season" text,
  "playoffs" text,
  "open_cup" text
);

Question: How many years was there a team that was part of the usisl pro league?
SQL: SELECT COUNT("year") FROM "year_by_year" WHERE "league"='usisl pro league';