CREATE TABLE table_54058 ("Name" TEXT, "Tenure Begin" TEXT, "Tenure End" FLOAT, "British monarch" TEXT, "Czechoslovak president" TEXT)

Who was the President of Czechoslovakia when an ambassador's tenure began in 1950?
SELECT "Czechoslovak president" FROM table_54058 WHERE "Tenure Begin" = '1950'
SELECT "Czechoslovak president" FROM "table_54058" WHERE "Tenure Begin" = '1950'