be a|is often a|is usually a|is actually a|may be a|is known as a|can be described as|is definitely a|is mostly a|is a really|really is a|rrs really a|serves as a|is really|is the|could be a|works as a|is seen as a|is truly a|is regarded as a|truly a|truly|regarded as a|actually|is known as the|rrs often a|will be a|is an accomplished|should be a|has been a|can be|could be|genuinely|is|might be a|is often|may be|actually a|definitely a|is another|is actually||frequently|generally|is but one|genuinely a|usually|seriously a|might be} frequent {source|supply|resource|origin|supplier|reference|base|form|foundation|cause|useful