The student was interested in the use of so far in this context. Could - or should - it be replaced by hitherto? Dictionaries did not give much help in distinguishing between them: so far in COBUILD, 'means up until the the present point in a situation, story, etc.' while hitherto in COBUILD, 'is used to refer to something that has been happening until now').

A concordance of the two expressions from 2¼ million words of New Scientist indicated:

1. When a number of 'side meanings' (eg "So far as I know") are excluded, the instances of so far: (300+: a sample based on every tenth is shown below) greatly outnumber those for hitherto (11: all shown below).
2. For both expressions, the concordances revealed an association with the Perfect in approximately half the instances (17/31 of the sample for so far and 6/11 for hitherto) (the auxiliary 'have' is highlighted in red in both concordances). The concordances also show a strong association with the Past Participle for hitherto (4/11 citations), an association which seems to be a weaker for so far (3/31)
3. The two expressions could not be readily distinguished on the basis of position in the sentence: thus 3/11 instances of hitherto and 10/31 of so far are sentence-initial.
4. A distinctive minor pattern for so far is its collocation with the Non-perfect Present (3/31 citations.¹

Footnote
1Intuition suggests that hitherto is extremely unlikely to be used with the Non-perfecr Present. This is confirmed by a subsequent examination of 872 citations for hitherto from The Times and Guardian (size of corpus: 80+ million words).

 1 cking. Is this "science" in any sense hitherto accepted? In Experts in Uncertainty, Roger 
 2  not the ones to which academics have hitherto been accustomed. In politics it is often ne
 3 ut when it asked for quality that had hitherto been demanded only in the defence industry,
 4 its rules on scientific publications. Hitherto, each scientific publishing house was restr
 5 ey to robot vision systems which have hitherto existed only in the realms of science ficti
 6 eturned to power was a positive move. Hitherto, in-house committees had evaluated the nati
 7 nd, where the bulk of formations have hitherto occurred, or from college or university dep
 8 trate many of the cognitive abilities hitherto reserved only for language-trained chimpanz
 9 cose testing" to screen for diabetes. Hitherto they have been able to make available simpl
10  way to current painkillers, blocking hitherto unknown receptors in the brain.
11 y to naloxone suggests that there are hitherto unsuspected receptors in the brain. He beli


 1 ark has raised Pounds Sterling 10 000 so far and Racal Recorders is providing Goodson with
 2  if they stop the release of amyloid. So far, they have found five candidates - all of whi
 3 esign drugs that outwit the bacteria. So far, isoniazid (also known as INH) is the only dr
 4  of microwave scrubbers Cha has built so far, the reactors are measured in centimetres. Th
 5 c an properly call it an illness. But so far no type of gene, or combination of genes, has
 6 , Russia, Switzerland and the US, but so far none has received full funding. Another attra
 7 of the small clinical tests conducted so far in the US have shown gene therapies to be hig
 8 in Britain we seem to find difficult. So far, it has proved impossible to shift money save
 9 ins sceptical about Britain's efforts so far. The six Faraday centres already established 
10 l fluid. We also  have some evidence, so far purely theoretical, that anyone can form a su
11 o receive significant Western funding so far were nuclear scientists who might otherwise h
12  effect just over five years ago, has so far failed to have any significant effect on the 
13 aded in Japan, and the government has so far made no plans to ban these.
14 o point out that toxicity testing has so far been limited to a small number of amphibians 
15 n - a view which American courts have so far upheld. By contrast, medical ethicists remain
16  with nonlinear systems. Studies have so far tended to concentrate upon experimental setup
17 h he has been driving for seven hours so far, he is allowed by law to do another two hours
18  curve of the heavier cobalt isotope. So far, however, the newly launched satellite, the C
19 kers may also have done the brewing." So far the excavators have exposed only the brewery 
20 ety of rodents and sometimes humans.  So far, the only clear example of a neural differenc
21 , such as loss of nutrients, may not. So far, the scientific community has failed to answe
22 are prepared to address it seriously. So far the articles published have rather underplaye
23 hough the findings apply only to mice so far, there is no reason why they should not apply
24  what they have discovered about Otze so far. Because the body is so delicate, much of the
25 orward, which provided the best proof so far that a particular source is indeed a black ho
26  be removed more quickly. The results so far look promising and the pharmaceuticals compan
27 ymer molecules in an organic solvent. So far we have treated neutrons as particles, but it
28 ing statistics produced at the summit so far, he said that if every nation used energy as 
29 ty in the area is plausible, but that so far there is no evidence to support it.
30  costed to make a protective vaccine. So far, the protein has not been tested in any exper
31 aphies, and details of the medals won so far and the records for each sport. Another date